#include <cstdio>
#include <cstring>
#include <stack>
#include <queue>
#include <vector>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#define MAXN 10000+10
#define MAXM 40000+100
#define INF 10000000
using namespace std;
struct Edge
{
    int from, to, next;
};
Edge edge[MAXM];
int head[MAXN], edgenum;
int man[MAXN];
int low[MAXN], dfn[MAXN];
int dfs_clock;
int ebcno[MAXN], ebc_cnt;
stack<int> S;
bool Instack[MAXN];
vector<int> ebc[MAXN];
int N, M;//N个教室 M个管道
int total;//记录所有教室的总人数
void init()
{
    edgenum = 0;
    memset(head, -1, sizeof(head));
}
void addEdge(int u, int v)
{
    Edge E = {u, v, head[u]};
    edge[edgenum] = E;
    head[u] = edgenum++;
}
void getMap()
{
    int a, b;
    while(M--)
    {
        scanf("%d%d", &a, &b);
        addEdge(a, b);
        addEdge(b, a);
    }
}
void tarjan(int u, int fa)
{
    int v, flag = 0;
    low[u] = dfn[u] = ++dfs_clock;
    S.push(u); Instack[u] = true;
    for(int i = head[u]; i != -1; i = edge[i].next)
    {
        v = edge[i].to;
        if(v == fa && !flag)
        {
            flag = 1;
            continue;
        }
        if(!dfn[v])
        {
            tarjan(v, u);
            low[u] = min(low[u], low[v]);
        }
        else if(Instack[v])
            low[u] = min(low[u], dfn[v]);
    }
    if(low[u] == dfn[u])
    {
        ebc_cnt++;
        ebc[ebc_cnt].clear();
        for(;;)
        {
            v = S.top(); S.pop();
            Instack[v] = false;
            ebcno[v] = ebc_cnt;
            ebc[ebc_cnt].push_back(v);
            if(v == u) break;
        }
    }
}
void find_cut(int l, int r)
{
    dfs_clock = ebc_cnt = 0;
    memset(low, 0, sizeof(low));
    memset(dfn, 0, sizeof(dfn));
    memset(ebcno, 0, sizeof(ebcno));
    memset(Instack, false, sizeof(Instack));
    for(int i = l; i <= r; i++)
        if(!dfn[i]) tarjan(i, -1);
}
int ebc_man[MAXN];//记录每个EBC的人数
vector<int> G[MAXN];
void suodian()
{
    for(int i = 1; i <= ebc_cnt; i++)
    {
        G[i].clear(); int sum = 0;
        for(int j = 0; j < ebc[i].size(); j++)
            sum += man[ebc[i][j]];
        ebc_man[i] = sum;
    }
    for(int i = 0; i < edgenum; i+=2)//建图 一开始没加 i+=2 无语。。。
    {
        int u = ebcno[edge[i].from];
        int v = ebcno[edge[i].to];
        if(u != v)
            G[u].push_back(v), G[v].push_back(u);
    }
}
int ans;//最小差值
int num[MAXN];//统计子节点 权值之和
void DFS(int u, int fa)//求解 最小差值
{
     num[u] = ebc_man[u];
     for(int i = 0; i < G[u].size(); i++)
     {
         int v = G[u][i];
         if(v == fa) continue;
         DFS(v, u);
         num[u] += num[v];
     }
     ans = min(ans, abs(total - 2 * num[u]));//更新
}
void solve()
{
    find_cut(0, N-1);//找EBC
    if(ebc_cnt == 1)//只有一个EBC
    {
        printf("impossible\n");
        return ;
    }
    suodian();//缩点
    ans = INF;
    DFS(1, -1);//遍历整棵树
    printf("%d\n", ans);
}
int main()
{
    while(scanf("%d%d", &N, &M) != EOF)
    {
        total = 0;
        for(int i = 0; i < N; i++)
            scanf("%d", &man[i]), total += man[i];//统计总人数
        init();
        getMap();
        solve();
    }
    return 0;
}