#include <fstream>
#include <queue>
#include <vector>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
int n,m,x,y,i,j,d,pi,pf,dim[30003];
vector<pair<int,int>>s[30003];
queue<int>q;
int main()
{
    cin>>n>>m>>pi>>pf;
    for(i=1;i<=m;i++){
        cin>>x>>y>>d;
        s[x].push_back({y,d});
        s[y].push_back({x,-d});
    }
    q.push(pi);
    while(!q.empty()){
        int nod=q.front();
        q.pop();
        for(i=0;i<s[nod].size();i++){
            int nod2=s[nod][i].first;
            if(dim[nod2]==0){
                q.push(nod2);
                dim[nod2]=dim[nod]+s[nod][i].second;
            }
        }
    }
    cout<<dim[pf];
    return 0;
}