#include<cstdio>
#include<algorithm>
#include<cstring>
#include<queue>
using namespace std;
int gi()
{
    int x=0,w=1;char ch=getchar();
    while ((ch<'0'||ch>'9')&&ch!='-') ch=getchar();
    if (ch=='-') w=0,ch=getchar();
    while (ch>='0'&&ch<='9') x=(x<<3)+(x<<1)+ch-'0',ch=getchar();
    return w?x:-x;
}
const int N = 1005;
const int inf = 1e9;
struct edge{int to,nxt,w;}a[N*20];
int n,m,S,T,head[N],cnt,dep[N],cur[N],vis1[N],vis2[N],cnt1,cnt2;
queue<int>Q;
void init()
{
    memset(head,0,sizeof(head));
    memset(vis1,0,sizeof(vis1));
    memset(vis2,0,sizeof(vis2));
    cnt=1;cnt1=cnt2=0;
}
void link(int u,int v,int w)
{
    a[++cnt]=(edge){v,head[u],w};
    head[u]=cnt;
    a[++cnt]=(edge){u,head[v],w};
    head[v]=cnt;
}
bool bfs()
{
    memset(dep,0,sizeof(dep));
    dep[S]=1;Q.push(S);
    while (!Q.empty())
    {
        int u=Q.front();Q.pop();
        for (int e=head[u];e;e=a[e].nxt)
            if (a[e].w&&!dep[a[e].to])
                dep[a[e].to]=dep[u]+1,Q.push(a[e].to);
    }
    return dep[T];
}
int dfs(int u,int f)
{
    if (u==T) return f;
    for (int &e=cur[u];e;e=a[e].nxt)
        if (a[e].w&&dep[a[e].to]==dep[u]+1)
        {
            int tmp=dfs(a[e].to,min(a[e].w,f));
            if (tmp) {a[e].w-=tmp;a[e^1].w+=tmp;return tmp;}
        }
    return 0;
}
void Dinic()
{
    while (bfs())
    {
        for (int i=1;i<=n;++i) cur[i]=head[i];
        while (dfs(S,inf)) ;
    }
}
void dfs1(int u)
{
    ++cnt1;
    for (int e=head[u];e;e=a[e].nxt)
        if (!vis1[a[e].to]&&a[e].w)
            vis1[a[e].to]=1,dfs1(a[e].to);
}
void dfs2(int u)
{
    ++cnt2;
    for (int e=head[u];e;e=a[e].nxt)
        if (!vis2[a[e].to]&&a[e^1].w)
            vis2[a[e].to]=1,dfs2(a[e].to);
}
int main()
{
    while (scanf("%d%d%d%d",&n,&m,&S,&T),n+m+S+T)
    {
        init();
        for (int i=1;i<=m;++i)
        {
            int u=gi(),v=gi(),w=gi();
            link(u,v,w);
        }
        Dinic();
        vis1[S]=vis2[T]=1;
        dfs1(S);dfs2(T);
        puts(cnt1+cnt2==n?"UNIQUE":"AMBIGUOUS");
    }
    return 0;
}