#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define TITLE "sate"

ifstream f (TITLE".in");
ofstream g (TITLE".out");

int n,m,StartNode,EndNode;

void bfs(int StartNode,vector<vector<int>> &Graph, vector<int> &Visited)
{
    Visited[StartNode]=1;
    queue<int>Q;
    for(Q.emplace(StartNode); !Q.empty(); Q.pop())
    {
        int CurrentNode=Q.front();
        for(int i=1; i<=n; i++)
        {
            if(Visited[i]==0 && Graph[CurrentNode][i]!=0)
            {
                Visited[i]=Visited[CurrentNode]+Graph[CurrentNode][i];
                Q.emplace(i);
            }
        }
    }
}

int main()
{
    f>>n>>m>>StartNode>>EndNode;
    vector<vector<int>> Graph(n+1,vector<int>(n+1,0));
    while(m--)
    {
        int a,b,c;
        f>>a>>b>>c;
        if(a>b)
            swap(a,b);
        Graph[a][b]=c;
        Graph[b][a]=-c;
    }
    vector<int>Answer(n+1,0);
    bfs(StartNode,Graph,Answer);
    g<<Answer[EndNode]-1;
    return 0;
}