#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
vector <pair<int,int> >v[100025];
queue <int> q;
int s,n,m,x,y,i,z,t,cost,d[100025],use[100025];
void bfs(int s)
{q.push(s);
d[s]=1;
while(!q.empty())
    {int nod=q.front();
    q.pop();
    for(pair<int,int> i:v[nod])
        if(!d[i.second])
            {q.push(i.second);
            d[i.second]=d[nod]+i.first;
            }
        
    }
}


int main()
{f>>n>>m>>x>>y;
for(i=1;i<=m;i++)
    {f>>z>>t>>cost;
    v[z].push_back(make_pair(cost,t));
    v[t].push_back(make_pair(-cost,z));
    }
bfs(x);
g<<d[y]-1;
}