#include <bits/stdc++.h>
#define Nmax 30005
#define inf 1<<30

using namespace std;

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

vector < pair<int,int> > mat[Nmax];
int n,m,x,y,a,b,c,d[Nmax],viz[Nmax];

void dfs(int vf)
{
    viz[vf]=1;
    for(int i=0;i<mat[vf].size();i++)
    {
        if(!viz[mat[vf][i].first])
        {
            d[mat[vf][i].first]=d[vf]+mat[vf][i].second;
            dfs(mat[vf][i].first);
        }
    }
}

int main()
{
    fin>>n>>m>>x>>y;
    for(;m--;)
    {
        fin>>a>>b>>c;
        mat[a].push_back({b,c});
        mat[b].push_back({a,-c});
    }
    dfs(x);
    fout<<d[y];
    return 0;
}