#include <cstdio>
#include <cstdlib>
#include <queue>
#include <vector>
#include <algorithm>
using namespace std;

const int NMAX = 30010;

int N, M, X, Y, Dist[NMAX], A, B, C;
vector<pair<int, int> > G[NMAX];

void BFS()
{
    for(int i = 1; i <= N; ++ i) Dist[i] = -1;
    
    queue<int> Q;
    Q.push(X);
    Dist[X] = 0;
    
    while(!Q.empty())
    {
        int Node = Q.front();
        Q.pop();
        
        for(vector<pair<int, int> > :: iterator it = G[Node].begin(); it != G[Node].end(); ++ it)
            if(Dist[it -> first] == -1)
            {
                Dist[it -> first] = Dist[Node] + (it -> first < Node ? -it -> second : it -> second);
                Q.push(it -> first);
            }
    }
    
    printf("%i\n", Dist[Y]);
}

int main()
{
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    
    scanf("%i %i %i %i", &N, &M, &X, &Y);
    for(int i = 1; i <= M; ++ i)
    {
        scanf("%i %i %i", &A, &B, &C);
        G[A].push_back(make_pair(B, C));
        G[B].push_back(make_pair(A, C));
    }
    
    BFS();
}