#include<bits/stdc++.h>
#define int long long
using namespace std;
int num,di[310],d[310][310],n,m,s,t,dd[310],ru[310],dis[310],vis[310],lj[310][310];
void ed(){printf("impossible");exit(0);}
signed main(){
	scanf("%lld%lld%lld%lld",&n,&m,&s,&t);
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++)d[i][j]=1e9;
		d[i][i]=0;
	}
	for(int i=1,u,v,w;i<=m;i++){
		scanf("%lld%lld%lld",&u,&v,&w);
		d[u][v]=d[v][u]=w;ru[u]++;ru[v]++;
	}
	for(int k=1;k<=n;k++){
		for(int i=1;i<=n;i++){
			for(int j=1;j<=n;j++){
				d[i][j]=min(d[i][j],d[i][k]+d[k][j]);
			}
		}
	}
	if(d[s][t]==1e9)ed();
	for(int i=1;i<=n;i++)for(int j=1;j<=n;j++)if(i!=j)di[i]=max(di[i],d[i][j]);
	do{
		num=0;
		for(int i=1;i<=n;i++){
			if(vis[i])continue;
			int fl=1;
			if(ru[i]==1){vis[i]=1;num++;continue;}
			for(int j=1;j<=n;j++)if(i!=j&&di[i]==d[i][j]){
				if(!vis[j])fl=0;
			}num+=fl;
			if(fl){vis[i]=1;
				for(int j=1;j<=n;j++)if(i!=j&&di[i]==d[i][j]&&vis[j])dis[i]=max(dis[i],dis[j]+d[i][j]);
			}
		}
	}while(num);
	if(vis[t])printf("%lld",dis[t]+d[s][t]);
	else ed();
}