#include<fstream>
using namespace std;

ifstream cin("sate.in");
ofstream cout("sate.out");

int a[100001],b[100001],c[100001],rasp[100001];

int main()
{
    int n,m,a1,b1,i;
    cin>>n>>m>>a1>>b1;rasp[a1]=1;
    for(i=1;i<=m;i++){
        cin>>a[i]>>b[i]>>c[i];
    }
    while(!rasp[b1]){
        for(i=1;i<=m;i++){
            if(!rasp[a[i]] && rasp[b[i]]){
                rasp[a[i]]=rasp[b[i]]-c[i];
            }else if(rasp[a[i]] && !rasp[b[i]]){
                rasp[b[i]]=rasp[a[i]]+c[i];
            }

        }
    }
    cout<<rasp[b1]-1;
    return 0;
}