//#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
vector<pair<int,int> > v[30005];
int q[30005];
int d[30005],sz=1,cur=1;
int main()
{
    int n,m,x,y,a,b,di;
    cin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++){
        cin>>a>>b>>di;
        v[a].push_back(make_pair(b,di));
        v[b].push_back(make_pair(a,-di));
    }
    for(int i=1;i<=n;i++){
        d[i]=30000000;
    }
    q[1]=x;
    d[x]=0;
    while(cur<=sz){
        a=q[cur];
        cur++;
        for(auto u:v[a]){
            if(d[u.first]==30000000){
                d[u.first]=d[a]+u.second;
                if(u.first==y){
                    cout<<d[y];
                    return 0;
                }
                q[++sz]=u.first;
            }
        }
    }
    return 0;
}