#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

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

int n,m,x,y,dst[20003][20003];
bool b[30002],ex;
vector <int> v[30002];

void dfs(int xc,int d){
    if(ex){
        return;
    }
    if(xc==y){
        fout<<d<<"\n";
        ex=true;
        return;;
    }
    for(int i=0;i<v[xc].size();i++){
        if(!b[v[xc][i]]){
            b[v[xc][i]]=true;
            dfs(v[xc][i],d+dst[xc][v[xc][i]]);

        }

    }
}

int main()
{
    fin>>n>>m>>x>>y;
    int ax,ax2,dx;
    for(int i=0;i<m;i++){
        fin>>ax>>ax2>>dx;
        dst[ax][ax2]=dx;
        dst[ax2][ax]=-dx;
        v[ax].push_back(ax2);
        v[ax2].push_back(ax);
    }
    dfs(x,0);
}