#include <fstream>
#include<bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
const int Nmax=30000;
typedef pair<int,int>pii;
vector<pii>graph[Nmax+5];
int viz[Nmax+5];
queue<pii>q;
int main()
{
    int n,m,x,y,Lastsum,sum=0,u,v,cost;
    fin>>n>>m>>x>>y;
    while(m--)
    {
        fin>>u>>v>>cost;graph[u].push_back({v,cost});
        graph[v].push_back({u,-cost});
    }
    q.push({x,0});
    while(!q.empty())
    {
        auto[nod,sum]=q.front();
        if(nod==y){Lastsum=sum;break;}
        for(auto x:graph[nod])
        {
            int node=x.first;

            if(!viz[node])
            {
                viz[node]=1;
                q.push({node,sum+x.second});
            }
        }
        q.pop();
    }
    fout<<Lastsum;
    return 0;
}