#include<fstream>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int a,b,i,N,M,sol[1<<15],x[1<<17],y[1<<17],c[1<<17];
int main()
{
    f>>N>>M>>a>>b;
    for(i=sol[a]=1;i<=M;++i) f>>x[i]>>y[i]>>c[i];
    while(!sol[b])
        for(i=1;i<=M;++i)
        {
            if(sol[x[i]]&&!sol[y[i]]) sol[y[i]]=sol[x[i]]+c[i];
            else if(!sol[x[i]]&&sol[y[i]]) sol[x[i]]=sol[y[i]]-c[i];
        }
    g<<sol[b]-1;
    return 0;
}