#include <fstream>
#include <vector>

using namespace std;

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

int n,m,i,x,y,x1,y1,cost,sol,s,ok,v[30005],d[30005],c[30005];
vector< pair<int, int> > L[30005];

int main()
{
    fin >> n >> m >> x >> y;
    for (i=1; i<=m; i++)
    {
        fin >> x1 >> y1 >> cost;
        L[x1].push_back(make_pair(y1, cost));
        L[y1].push_back(make_pair(x1, -cost));
    }
    int p = 1; int u = 1; c[1] = x; v[x] = 1;
    while (p <= u)
    {
        int nod = c[p];
        for (i=0; i<L[nod].size(); i++)
        {
            int vecin = L[nod][i].first; int cost = L[nod][i].second;
            if (!v[vecin])
            {
                v[vecin] = 1; c[++u] = vecin;
                d[vecin] = d[nod]+cost;
            }
        }
        p++;
    }
    fout << d[y];
    return 0;
}