#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;

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

int n, m, X, Y, d[30001];
bool v[30001];
vector<pair<int, int>> L[30001];

int main()
{
    int i, j, D;
    queue<int> q;

    fin >> n >> m >> X >> Y;
    while (m--) {
        fin >> i >> j >> D;
        if (i > j) {
            swap(i, j);
        }
        L[i].push_back({j, D});
        L[j].push_back({i, -D});
    }

    q.push(X);
     while (!q.empty()) {
        i = q.front();
        q.pop();
        if (i == Y) {
            fout << d[Y] << "\n";
            return 0;
        }
        if (v[i]) continue;
        v[i] = 1;
        for (auto k : L[i]) {
            if (!v[k.first]) {
                d[k.first] = d[i] + k.second;
                q.push(k.first); 
            }
        }
    }

    return 0;
}