#include <cstdio>
#include <vector>
using namespace std;
const int NMAX = 30005;
bool viz[NMAX];
int dist[NMAX];
vector <pair<int, int>> vec[NMAX];
void dfs(int u) {
    viz[u] = 1;
    for(auto v : vec[u]) {
        if(viz[v.first] == 0) {
            dist[v.first] = dist[u] + v.second;
            dfs(v.first);
        }
    }
}

int main() {
    int n, m, x, y;
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);
    scanf("%d%d%d%d", &n, &m, &x, &y);
    for(int i = 1; i <= m; i++) {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        if(a > b) {
            c = -c;
        }
        vec[a].push_back({b, c});
        vec[b].push_back({a, -c});
    }
    dfs(x);
    if(dist[y] < 0) {
        dist[y] = -dist[y];
    }
    printf("%d\n", dist[y]);
    return 0;
}