#include <cstdio>
#include <algorithm>
#include <cstring>
#include <vector>
#include <deque>
#define Nmax 30002
using namespace std;
int dist[Nmax], n, m, x, y;
vector < pair < int,int > > V[Nmax];
deque < int > q;
void read()
{
    int i, a, b, d;
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    scanf("%d %d %d %d", &n, &m, &x, &y);
    for (i = 1; i <= m; ++ i)
    {
        scanf("%d %d %d", &a, &b, &d);
        V[a].push_back(make_pair(b, d));
        V[b].push_back(make_pair(a, d));
    }
}
void bfs()
{
    int i, node;
    for (i = 1; i <= n; ++ i)
        dist[i] = -1;
    q.push_back(x);
    dist[x] = 0;
    while (!q.empty())
    {
        node = q.front();
        q.pop_front();
        for (i = 0; i < V[node].size(); ++ i)
            if (dist[V[node][i].first] == -1)
        {
            if (V[node][i].first > node)
               dist[V[node][i].first] = dist[node] + V[node][i].second;
            else
                dist[V[node][i].first] = dist[node] - V[node][i].second;
            q.push_back(V[node][i].first);
        }
    }
}
void write()
{
    bfs();
    printf("%d", dist[y]);
}
int main()
{
    read();
    write();
    return 0;
}