#include <bits/stdc++.h>
using namespace std;

#define NMAX 30001

int n, m, from, target;
int64_t d[NMAX];
vector<pair<int, int>> g[NMAX];
priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>>pq;
int main(){
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    cin>>n>>m>>from>>target;
    fill(d, d+n+1, LLONG_MAX);
    for (int i=0; i<m; ++i){
        int x, y, c;
        cin>>x>>y>>c;
        g[x].push_back({y, c});
        g[y].push_back({x, -c});
    }
    d[from]=0;
    pq.push({0, from});
    while(!pq.empty()){
        int i=pq.top().second, t=pq.top().first;
        pq.pop();
        if (t>d[i])continue;
        for (auto it:g[i]){
            if (d[it.first]>d[i]+it.second){
                d[it.first]=d[i]+it.second;
                pq.push({d[it.first], it.first});
            }
        }
    }
    cout<<(d[target]>=0 ? d[target] : -d[target]);
    return 0;
}