#include<fstream>
using namespace std;
ifstream in("sate.in");
ofstream out("sate.out");
int N,M,x,y,s[30003],a[100030],b[100030],d[100030];
int main()
{
    in>>N>>M>>x>>y;
    s[x]=1;
    for(int i=1; i<=M; i++)
        in>>a[i]>>b[i]>>d[i];
    while(!s[y])
    {
        for(int i=1; i<=M; i++)
        {
            if(s[a[i]]&&!s[b[i]])s[b[i]]=s[a[i]]+d[i];
            else if(!s[a[i]]&&s[b[i]])s[a[i]]=s[b[i]]-d[i];
        }
    }
    out<<s[y]-1<<"\n";
    return 0;
}