#include <fstream>
#include <vector>
#include <queue>
#define nmax 30001
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
int n,m,cost[nmax],a,b,x,y,c;
vector<pair<int,int>>v[nmax];
queue<int>q;
int main()
{
    cin>>n>>m>>a>>b;
    for(int i=1;i<=m;i++){
        cin>>x>>y>>c;
        v[x].push_back({y,c});
        v[y].push_back({x,-c});
    }
    q.push(a);
    while(!q.empty()){
        int nod=q.front();
        q.pop();
        for(auto i:v[nod])
            if(cost[i.first]==0){
                cost[i.first]=cost[nod]+i.second;
                q.push(i.first);
            }
    }
    cout<<cost[b];
    return 0;
}