#include <fstream>
#include <vector>
#define NMAX 30002
using namespace std;

int n, m;
int x, y;
int dist[NMAX];
vector<int> q;
vector<int> v[NMAX];
vector<int> c[NMAX];
char viz[NMAX];


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

void read() {
    in>>n>>m>>x>>y;
    int a, b, cost;
    if (x > y) swap(x, y);

    for (int i = 1; i <= m; i++) {
        in>>a>>b>>cost;
        v[a].push_back(b);
        c[a].push_back(cost);
        v[b].push_back(a);
        c[b].push_back(cost);
    }
}

void bfs() {
    q.push_back(x);
    viz[x] = 1;
    int sat, lim, vecin;
    int st = 0;
    int dr = 0;
    while (st <= dr && !viz[y]) {
        sat = q[st];
        lim =  v[sat].size();
        for (int i = 0; i < lim; i++) {
             vecin = v[sat][i];
            if (!viz[vecin]) {
                viz[vecin] = 1;
                q.push_back(vecin);
                dr++;
                if (vecin > sat) {
                    dist[vecin] = dist[sat] + c[sat][i];
                } else {
                    dist[vecin] = dist[sat] - c[sat][i];
                }
            }
        }
        st++;
    }
}

int main() {
    read();
    bfs();
    out<<dist[y];
    return 0;
}