#include <cstdio>
#include <vector>
#include <queue>
#include <cstring>
#define NMAX 30002

using namespace std;

int i, n, m, X, Y, x, y, dist, way[NMAX];

vector< pair < int, int> > v[NMAX];
queue <int> q;

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

    scanf("%d %d %d %d", &n, &m, &X, &Y);

    for (i=1; i<=m; ++i)
    {
         scanf("%d %d %d", &x, &y, &dist);
        v[x].push_back(make_pair(y,dist));
        v[y].push_back(make_pair(x,-dist));
    }
    q.push(X);

  //  memset(way, -1, sizeof(way));
    way[X] = 1;

    while (!q.empty())
    {
        int a = q.front();
        q.pop();

        int nn = v[a].size();
        for (vector < pair <int, int> > :: iterator it = v[a].begin(); it != v[a].end(); ++it)
            if (way[it -> first] == 0)
        {
            way[it -> first] = way[a] + it -> second;
            q.push(it -> first);
        }
    }

    printf("%d", way[Y]-1);
    return 0;
}