#include <bits/stdc++.h>

using namespace std;

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

vector <pair <int,int>> gr[30005];
int vis[30005];

void DFS(int node,int loc,int &ans,int y) {
	if (node==y){
		ans = loc;
	}
	vis[node] = 1;
	for (auto x:gr[node]){
		if (vis[x.first]) continue;
		DFS(x.first,loc+x.second,ans,y);
	}
}

int main()
{
    int n,m,x,y;
    fin >> n >> m >> x >> y;
    for (int i=1;i<=m;++i){
        int I,J,D;
        fin >> I >> J >> D;
        gr[I].push_back({J,D});
        gr[J].push_back({I,-D});
    }
    int ans = 0;
    DFS(x,0,ans,y);
    fout << ans;
    return 0;
}