#include<bits/stdc++.h>
using namespace std;
const int N=1e4+5, M=2e4+5;
struct edge{
	int to, nt;
}E[M<<1];
int head[N], sz[N], dfn[N], low[N], id, tot, ans;
void dfs(int u, int f){
	dfn[u]=low[u]=++id;
	for(int i=head[u]; ~i; i=E[i].nt){
		int &v=E[i].to;
		if(!dfn[v]){
			dfs(v, i>>1);
			low[u]=min(low[v], low[u]);
			sz[u]+=sz[v];
			if(low[v]>dfn[u]){
				ans=min(ans,abs(tot-2*sz[v]));
			}
		}
		else if(v!=i>>1&&dfn[v]<dfn[u]){
			low[u]=min(low[u], dfn[v]);
		}
	}
}
int main(){
	//freopen("in", "r", stdin);
	int n, m, u, v;
	for(;~scanf("%d%d", &n, &m);){
		ans=INT_MAX, tot=0;
		memset(head, -1, sizeof(head)), id=0;
		for(int i=0; i<n; i++) scanf("%d", sz+i), tot+=sz[i];
		for(;m--;){
			scanf("%d%d", &u, &v);
			E[id]={v, head[u]}, head[u]=id++;
			E[id]={u, head[v]}, head[v]=id++; 
		}
		memset(dfn, 0, sizeof(dfn)), id=0;
		dfs(0, -1);
		ans==INT_MAX?puts("impossible"):printf("%d\n", ans);
	}
}