#include <cstdio>
#include <vector>
#include <queue>

using namespace std;

const int NMAX = 30001;
struct muchie {
    int v, c;
};

vector<muchie> a[NMAX];
queue<int> q;
int d[NMAX],
    n, m, x, y;

void bfs() {
    q.push(x);
    d[x] = 1;
    int crt;
    while(!q.empty()) {
        crt = q.front();
        q.pop();
        for(int i = 0; i < a[crt].size(); i++)
            if(d[a[crt][i].c] == 0) {
                d[a[crt][i].v] = d[crt] + a[crt][i].c;
                q.push(a[crt][i].v);
            }
    }
}
int main()
{
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);

    scanf("%i %i %i %i", &n, &m, &x, &y);

    while(m--) {
        int i, j, D;
        scanf("%i %i %i", &i, &j, &D);
        muchie z;
        z.v = j; z.c = D;
        a[i].push_back(z);
        z.v = i; z.c = -D;
        a[j].push_back(z);
    }

    bfs();

    printf("%i", d[y]-1);
    return 0;
}