#include <iostream>
#include <fstream>
#include <deque>
#include <unordered_map>
#include <vector>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n, m, s, a, b, x, y, c[30002], ct;
vector< pair<int, int> > lv[30002];
vector< pair<int, int> >::iterator it;
deque<int> q;
void bf(){
while(!q.empty()){
        x=q.front();
    for(it=lv[x].begin(); it!=lv[x].end(); ++it)
        if(!c[it->first]){
        c[it->first]=c[x]+it->second;
        q.push_back(it->first);
        if(it->first==y) {g<<c[y]-1;return;}
    }
    q.pop_front();
}
}

int main()
{
f>>n>>m>>x>>y;
while(m--)
    f>>a>>b>>ct,
    lv[a].push_back(make_pair(b, ct)),
    lv[b].push_back(make_pair(a, -ct));
q.push_back(x); c[x]=1;
bf();
    return 0;
}