#include <fstream>
#include <vector>

using namespace std;

ifstream cin("sate.in");
ofstream cout("sate.out");

#define MAXN 100000
vector<pair<int, int>> mat[MAXN+1];
int dist[MAXN+1];
int n, m, xi, yi;

void dfs(int poz, int val) {
    dist[poz]=val;
    for(auto i:mat[poz])
        if(dist[i.first]==0)
            dfs(i.first, val+i.second);
}

int main() {
    cin>>n>>m>>xi>>yi;
    int i, x, y, d;
    for(i=0; i<m; i++) {
        cin>>x>>y>>d;
        mat[x].push_back({y, d});
        mat[y].push_back({x, -d});
    }
    dfs(xi, 0);
    cout<<dist[yi];
}