#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<vector>
using namespace std;
#pragma comment(linker, "/STACK:102400000,102400000")
#define ls i<<1
#define rs ls | 1
#define mid ((ll+rr)>>1)
#define pii pair<int,int>
#define MP make_pair
typedef long long LL;
const long long INF = 1e18;
const double Pi = acos(-1.0);
const int N = 5e4+10, M = 4e4+10, mod = 1e9+7, inf = 2e9;
int all;
int vis[N];
int n,m,a[N],vv[N],uu[N];
int head[M],belong[N],inq[N];
int dfn[N],low[N],scc,siz[N],tot,ans,top,t,hav[N],q[N];
vector<int > G[N];
void init() {
        scc = 0;
        all = 0;
        tot = 0;
        top = 0;
        t = 0;
        memset(head,-1,sizeof(head));
        memset(dfn,0,sizeof(dfn));
        memset(hav,0,sizeof(hav));
}
struct node{int to,next,id;}e[N * 4];

void dfs(int u,int fa) {
        dfn[u]=low[u]=++tot;
        q[++top] = u;
        inq[u] = 1;
        for(int i = head[u]; i != -1; i = e[i].next) {
            int to = e[i].to;
            if(e[i].id) continue;
            e[i].id = e[i^1].id = 1;
            if(!dfn[to]) {
                dfs(to,u);
                low[u] = min(low[u],low[to]);
            } else if(inq[to]) low[u] = min(low[u],dfn[to]);
        }
        if(low[u] == dfn[u]) {
            scc++;
            do{
                inq[q[top]] = 0;
                belong[q[top]]=scc;
                hav[scc]+=a[q[top]];
            }while(u!=q[top--]);
        }
}

void add(int u,int v) {
        e[t].to=v;
        e[t].next=head[u];
        e[t].id = 0;
        head[u]=t++;
}
void Tarjan() {
        for(int i = 1; i <= n; ++i) {
            if(!dfn[i])
            {
                dfs(i,-1);
            }
        }
}
void dfs2(int u,int fa) {
        siz[u] = hav[u];
        for(int i = 0; i < G[u].size(); ++i) {
            int to = G[u][i];
            if(to == fa) continue;
            dfs2(to,u);
            siz[u] += siz[to];
            ans = min(ans,abs(all - siz[to] - siz[to]));
        }
}

int main() {
        while(~scanf("%d%d",&n,&m)) {
            init();
            for(int i = 1; i <= n; ++i) scanf("%d",&a[i]),all += a[i];
            for(int i = 1; i <= m; ++i) {
                int u,v;
                scanf("%d%d",&u,&v);
                u++,v++;
                add(u,v),add(v,u);
                uu[i] = u;
                vv[i] = v;
            }
            Tarjan();
            //cout<<scc<<endl;
           // return 0;
            for(int i = 0; i <= scc; ++i) G[i].clear();
            for(int i = 1; i <= m; ++i) {
                int u = belong[uu[i]];
                int v = belong[vv[i]];
                if(u == v) continue;
                G[u].push_back(v);
                G[v].push_back(u);
            }
            ans = inf;
            dfs2(1,0);
            if(ans == inf) puts("impossible");
            else
            printf("%d\n",ans);
        }
        return 0;
}