#include <bits/stdc++.h>
using namespace std;
int nodes, edges, X, Y, u, v, cost, visited[30001];
vector<pair<int, int> > adj[30001];
int BFS(int start)
{
    queue<pair<int, int> > q; q.push({start, 0});
    while(!q.empty())
    {
        int current = q.front().first,
            cost    = q.front().second;
        if(current == Y) return cost;
        q.pop();
        for(auto child : adj[current])
        {
            if(!visited[child.first])
            {
                q.push({child.first, child.second + cost});
                visited[child.first] = 1;
            }
        }
    }
}
int main()
{
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    scanf("%d %d %d %d", &nodes, &edges, &X, &Y);
    for(int i = 1; i <= edges; i++)
    {
        scanf("%d %d %d", &u, &v, &cost);
        adj[u].push_back({v,  cost});
        adj[v].push_back({u, -cost});
    }
    printf("%d", BFS(X));
    return 0;
}