#include <stdio.h>
#include <vector>
#include <queue>
#define pb push_back
#define mp make_pair
using namespace std;

vector < pair <int, int> > g[30001];
queue <int> q;
int n,m,a,b,x,y,d,i,s,way[30001];
bool sel[30001];
void bf (int x)
{
    int nod,i,nurm,c;
    q.push(x); sel[x]=true;way[x]=0;
    while (!q.empty())
    {
       nod=q.front();
       for (i=0;i<g[nod].size();i++)
        {
           nurm=g[nod][i].first;
           c=g[nod][i].second;
           if ( !sel[nurm])
            {
              q.push(nurm);
              sel[nurm]=true;
              way[nurm]=way[nod]+c;
              if (nurm==b) return;
            }
        }
        q.pop();
     }
}
int main()
{
freopen("sate.in","r",stdin);
freopen("sate.out","w",stdout);

scanf("%d%d%d%d",&n,&m,&a,&b);
     for (i=1;i<=m;i++)
     {
         scanf("%d%d%d\n",&x,&y,&d);
         g[x].pb(mp(y,d));
         g[y].pb(mp(x,-d));
     }
bf(a);
if (way[b]!=0) printf("%d\n",way[b]); else printf("-1\n");
return 0;
}