#include <iostream>
#include<fstream>
#include<vector>
using namespace std;ifstream fin("sate.in");ofstream fout("sate.out");struct qer{int x,y;};vector<qer>v[30001];
int n,m,x,y,i,val,st,dr,dist[30001],rsp;bool mark[30001];
void dfs(int poz,int dst){
    mark[poz]=1;if(poz==y)rsp=dst;
    for(int j=0;j<v[poz].size();j++)if(!mark[v[poz][j].x])dfs(v[poz][j].x,dst+v[poz][j].y);
}
int main()
{
    fin>>n>>m>>x>>y;for(i=1;i<=m;i++){fin>>st>>dr>>val;v[st].push_back({dr,val});v[dr].push_back({st,-val});}dfs(x,0);fout<<rsp;
    return 0;
}