#include <iostream>
#include <fstream>
#include <queue>
#include <vector>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
vector<pair<int,int> > a[30005];
queue<int> q;
int n,m,x,y,k,i,j,dist,viz[30005];
void bfs(int p)
{
    q.push(p);
    viz[p]=1;
    while(!q.empty())
    {
        int t=q.front();
        for(int i=0;i<a[t].size();i++)
        {
            int s=a[t][i].first;
            int d=a[t][i].second;
            if(!viz[s])
            {
                q.push(s);
                viz[s]=viz[t]+d;
            }
        }
        q.pop();
    }
}
int main()
{
    f>>n>>m>>x>>y;
    for(k=1;k<=m;k++)
    {
        f>>i>>j>>dist;
        a[i].push_back({j,dist});
        a[j].push_back({i,-dist});
    }
    bfs(x);
    g<<viz[y]-1;
    return 0;
}