#include <bits/stdc++.h>

using namespace std;

ifstream fin ("teatru.in");
ofstream fout ("teatru.out");

vector <pair <int,int> >h[30001];
queue <int> q;
int d[30001], X, Y, n;

///d[i] - distanta de la x la i

int main()
{
    int i, m, x, y, dist, j;
    cin >> n >> m >> X >> y;
    for(i = 1; i <= m; i++)
    {
        cin >> x >> y >> dist;
        h[x].push_back({y, dist});
        h[y].push_back({x, -dist});
    }
    for(i = 1; i <= n; i++)
        d[i] = -1;
    d[x] = 0;
    q.push(x);
    while(!q.empty())
    {
        x = q.front();
        q.pop();
        for(auto w : h[x])
        {
            j = w.first;
            dist = w.second;
            if(d[j] == -1)
            {
                d[j] = d[x] + dist;
                q.push(j);
                if(d[Y] >= 0) {cout << d[Y]; return 0;}
            }
        }

    }

    return 0;
}