#include<stdio.h>
using namespace std;
const int MU = 100024;
const int NR = 30005;
int nr = 0, vf[MU], lst[NR], urm[MU], c[MU],d[NR], viz[NR], q[NR], n, sfarsit;
void adauga(int x, int y, int cc)
{
    vf[++nr] = y;
    if (x < y)
        c[nr] = cc;
    else
        c[nr] = -cc;
    urm[nr] = lst[x];
    lst[x] = nr;
}
void bfs(int x0)
{
    int p = 1, i, u = 0, x, y, poz;
    for (i = 1; i <= n; i++)
        d[i] = -1;
    q[++u] = x0;
    d[x0] = 0;
    while (p <= u)
    {
        x  = q[p++];
        poz = lst[x];
        while (poz != 0)
        {
            y = vf[poz];
            if (d[y] == -1)
            {
                q[++u] = y;
                d[y] = d[x] + c[poz];
            }
            poz = urm[poz];
            if(d[sfarsit] != -1)
                return;
        }
    }
}
int main ()
{
    FILE *in, *out;
    in = fopen ("sate.in", "r");
    out = fopen ("sate.out", "w");
    int i, m, cc, x ,y, pornire;
    fscanf (in, "%d%d%d%d", &n, &m, &pornire, &sfarsit);
    for (i = 1; i <= m; i++)
    {
        fscanf (in, "%d%d%d", &x, &y, &cc);
        adauga(x, y, cc);
        adauga(y, x, cc);
    }
    bfs(pornire);
    fprintf(out, "%d", d[sfarsit]);
    return 0;
}