#include <fstream>
#include <algorithm>
#include <vector>
#include <queue>
using namespace std;

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

int n, m, x, y, dist[30005];
vector<pair<int, int>> adj[30005];

void bfs(int nod){
    queue<int> q;
    q.push(nod);
    while(!q.empty()){
        int nodCrt=q.front();
        q.pop();
        for(auto vecin:adj[nodCrt]){
            if(dist[vecin.first]==0){
                dist[vecin.first]=dist[nodCrt]+vecin.second;
                q.push(vecin.first);
            }
        }
    }
}

int main(){
    cin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++){
        int a, b, d;
        cin>>a>>b>>d;
        if(a>b){
            swap(a, b);
        }
        adj[a].push_back({b, d});
        adj[b].push_back({a, -d});
    }
    dist[x]=1;
    bfs(x);
    cout<<dist[y]-1;
}