#include <iostream>
#include <fstream>
#include <queue>
#include <cstdio>
#include <cstdlib>
using namespace std;

FILE *f = fopen("sate.in", "r");
FILE *g = fopen("sate.out","w");
vector <int> vf[30001], pond[30001];
int viz[30001], sum[30001];
int n, m, x,y;
char s[100];
void citeste()
{
    int a,b, pondere;
    fgets(s,100,f);
    sscanf(s, "%d%d%d%d", &n,&m,&x,&y);
    for(int i = 0; i < m; i++)
        {
            fgets(s, 100, f);
            sscanf(s,"%d%d%d", &a, &b, &pondere);
            //cout<<a<<' '<<b<<' '<<pondere<<'\n';
            vf[a].push_back(b);
            vf[b].push_back(a);
            pond[a].push_back(pondere);
            pond[b].push_back(pondere);
        }
}
void bf(int rad)
{
    queue <int> coada;
    coada.push(rad);
    viz[rad] = 1;
    while(!coada.empty() && sum[y] == 0)
    {
    rad = coada.front();
    coada.pop();
    int dim = vf[rad].size();
    for(int i = 0; i < dim && sum[y]==0; i++)
    {
        if(!viz[vf[rad][i]])
        {
            coada.push(vf[rad][i]);
            viz[vf[rad][i]] = 1;
            if(rad < vf[rad][i])
                sum[vf[rad][i]] += sum[rad] + pond[rad][i];
            else
                sum[vf[rad][i]] += sum[rad] - pond[rad][i];
            if (sum[y] > 0)
                return ;
        }
    }
    }
}
int main()
{
    citeste();
    bf(x);
    fprintf(g, "%d", sum[y]);
    return 0;
}