#include <bits/stdc++.h>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");

int N, M, X, Y, A, B, dist;

int BFS(int start ,vector<vector<pair<int, int>> > &L, vector<bool> &visited, int N)
{
    set<pair<int,int> > Q;

    Q.insert(make_pair(start,0)); visited[start] = true;
    while(!Q.empty())
    {
        int nod_current = (*Q.begin()).first;
        int dist = (*Q.begin()).second;
        Q.erase(Q.begin());
        for(auto neighbour : L[nod_current])
        {
            if(!visited[neighbour.first])
            {
                if(neighbour.first == Y)
                    return dist + neighbour.second;
                visited[neighbour.first] = true;
                Q.insert(make_pair(neighbour.first, dist + neighbour.second));
            }
        }
    }
}

int main()
{
    f>>N>>M>>X>>Y;
    vector<vector<pair<int, int>> > L(N+1);
    for(int i = 1; i <= M; i++)
    {
        f>>A>>B>>dist;
        if(A < B)
        {
            L[A].push_back(make_pair(B, dist));
            L[B].push_back(make_pair(A, -dist));
        }
        else{
                L[A].push_back(make_pair(B, -dist));
                L[B].push_back(make_pair(A, dist));
        }
    }
    f.close();
    vector<bool> visited(N+1);
    g<<BFS(X,L,visited,N);
    g.close();
    return 0;
}