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