#include <stdio.h>
#include <vector>
#include <queue>
#define NMAX 30005
#define MMAX 100005

using namespace std;

vector< pair<int,int> > G[NMAX];
queue<int> Q;
bool viz[NMAX];
int N,M,X,Y,C=0,dist[NMAX];

void read()
{
    scanf("%d %d %d %d\n",&N,&M,&X,&Y);

    int i,a,b,c;
    for (i=1;i<=M;i++)
    {
        scanf("%d %d %d\n",&a,&b,&c);
        G[a].push_back(make_pair(b,c));
        G[b].push_back(make_pair(a,-c));
    }
}

void bfs()
{
    while(!Q.empty())
    {
        int nod=Q.front();
        Q.pop();
        for (int i=0;i<G[nod].size();i++)
            if (viz[G[nod][i].first]==0)
            {
                Q.push(G[nod][i].first);
                viz[G[nod][i].first]=1;
                dist[G[nod][i].first]=dist[nod]+G[nod][i].second;
                if (G[nod][i].first==Y)
                    return;
            }
    }

}

int main()
{
    freopen ("sate.in","r",stdin);
    freopen ("sate.out","w",stdout);

    read();
    Q.push(X);
    viz[X]=1;
    bfs();

    printf("%d\n",dist[Y]);

    fclose(stdin);
    fclose(stdout);
    return 0;
}