/*
ID: wuqi9395@126.com
PROG:
LANG: C++
*/
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<cmath>
#include<cstdio>
#include<vector>
#include<string>
#include<fstream>
#include<cstring>
#include<ctype.h>
#include<iostream>
#include<algorithm>
using namespace std;
#define INF (1<<30)
#define PI acos(-1.0)
#define mem(a, b) memset(a, b, sizeof(a))
#define rep(i, a, n) for (int i = a; i < n; i++)
#define per(i, a, n) for (int i = n - 1; i >= a; i--)
#define eps 1e-6
#define debug puts("===============")
#define pb push_back
#define mkp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
#define POSIN(x,y) (0 <= (x) && (x) < n && 0 <= (y) && (y) < m)
typedef long long ll;
typedef unsigned long long ULL;
const int maxn = 1000;
const int maxm = 222222;
int m;
struct node {
    int u;
    int v;    // vertex
    int cap;    // capacity
    int flow;   // current flow in this arc
    int nxt;
} e[maxm * 2];
int g[maxn], cnt;
int st, ed, n, N, M, A, B;
void add(int u, int v, int c) {
    e[++cnt].v = v;
    e[cnt].u = u;
    e[cnt].cap = c;
    e[cnt].flow = 0;
    e[cnt].nxt = g[u];
    g[u] = cnt;

    e[++cnt].v = u;
    e[cnt].u = v;
    e[cnt].cap = 0;
    e[cnt].flow = 0;
    e[cnt].nxt = g[v];
    g[v] = cnt;
}

void init() {
    memset(g, 0, sizeof(g));
    cnt = 1;
    int u, v, c;
    while(M--) {
        scanf("%d%d%d", &u, &v, &c);
        add(u, v, c);
        add(v, u, c);
    }
    st = A, ed = B;
    n = N + 3;
}

int dist[maxn], numbs[maxn], q[maxn];
void rev_bfs() {
    int font = 0, rear = 1;
    for (int i = 0; i <= n; i++) { //n为总点数
        dist[i] = maxn;
        numbs[i] = 0;
    }
    q[font] = ed;
    dist[ed] = 0;
    numbs[0] = 1;
    while(font != rear) {
        int u = q[font++];
        for (int i = g[u]; i; i = e[i].nxt) {
            if (e[i ^ 1].cap == 0 || dist[e[i].v] < maxn) continue;
            dist[e[i].v] = dist[u] + 1;
            ++numbs[dist[e[i].v]];
            q[rear++] = e[i].v;
        }
    }
}
int maxflow() {
    rev_bfs();
    int u, totalflow = 0;
    int curg[maxn], revpath[maxn];
    for(int i = 0; i <= n; ++i) curg[i] = g[i];
    u = st;
    while(dist[st] < n) {
        if(u == ed) {   // find an augmenting path
            int augflow = INF;
            for(int i = st; i != ed; i = e[curg[i]].v)
                augflow = min(augflow, e[curg[i]].cap);
            for(int i = st; i != ed; i = e[curg[i]].v) {
                e[curg[i]].cap -= augflow;
                e[curg[i] ^ 1].cap += augflow;
                e[curg[i]].flow += augflow;
                e[curg[i] ^ 1].flow -= augflow;
            }
            totalflow += augflow;
            u = st;
        }
        int i;
        for(i = curg[u]; i; i = e[i].nxt)
            if(e[i].cap > 0 && dist[u] == dist[e[i].v] + 1) break;
        if(i) {   // find an admissible arc, then Advance
            curg[u] = i;
            revpath[e[i].v] = i ^ 1;
            u = e[i].v;
        } else {    // no admissible arc, then relabel this vertex
            if(0 == (--numbs[dist[u]])) break;    // GAP cut, Important!
            curg[u] = g[u];
            int mindist = n;
            for(int j = g[u]; j; j = e[j].nxt)
                if(e[j].cap > 0) mindist = min(mindist, dist[e[j].v]);
            dist[u] = mindist + 1;
            ++numbs[dist[u]];
            if(u != st)
                u = e[revpath[u]].v;    // Backtrack
        }
    }
    return totalflow;
}
int vis[maxn];
vector<int> G[maxn];
int stcnt, edcnt;
void dfs1(int u) {
    stcnt++;
    vis[u] = 1;
    for (int i = g[u]; i; i = e[i].nxt) if (!vis[e[i].v] && e[i].cap) {
        dfs1(e[i].v);
    }
}
void dfs2(int u) {
    edcnt++;
    vis[u] = 1;
    for (int i = 0; i < G[u].size(); i++) if (!vis[G[u][i]]) dfs2(G[u][i]);
}
int main () {
    while(~scanf("%d%d%d%d", &N, &M, &A, &B), N || M || A || B) {
        init();
        maxflow(); //先做一次网络流得到最小割
        memset(vis, 0, sizeof(vis));
        stcnt = edcnt = 0;
        for (int i = 0; i <= N; i++) G[i].clear();
        for (int i = 2; i < cnt; i += 2) if (e[i].cap) {
            G[e[i].v].pb(e[i].u); //将非满流的边反向构图，以便从汇点dfs
        }
        dfs1(st); // 从源点dfs
        dfs2(ed); // 从汇点dfs
        if (stcnt + edcnt == N) puts("UNIQUE");
        else puts("AMBIGUOUS");
    }
    return 0;
}