#include <bits/stdc++.h>

using namespace std;

struct muchie{
    int nod,cost;
};

int n,m,X,Y,d[30003];
vector<muchie>L[30003];

inline void Citire()
{
    int i,x,y,c;
    muchie w;
    ifstream fin("sate.in");
    fin>>n>>m>>X>>Y;
    for(i=1;i<=m;++i)
    {
        fin>>x>>y>>c;
        w.nod=y;
        w.cost=c;
        L[x].push_back(w);
        w.nod=x;
        w.cost=-c;
        L[y].push_back(w);
    }
    fin.close();
}

inline void DFS(int x)
{
    unsigned int k;
    int y,c;
    for(k=0;k<L[x].size();++k)
    {
        y=L[x][k].nod;
        c=L[x][k].cost;
        if(d[y]==20000005)
        {
            d[y]=d[x] + c;
            DFS(y);
        }
    }
}

int main()
{
    int i;
    Citire();
    for(i=1;i<=n;++i)
        d[i]=20000005;
    d[X]=0;
    DFS(X);
    ofstream fout("sate.out");
    d[Y]=abs(d[Y]);
    if(d[Y]==20000005)
        while(1)
            cout<<".\n";
    fout<<d[Y]<<"\n";
    return 0;
}