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

ifstream f("sate.in");

vector <int> vf[30001], pond[30001];
int viz[30001], sum[30001];
int n, m, x,y;
void citeste()
{
    int a,b, pondere;
    f>>n>>m>>x>>y;
    for(int i = 0; i < m; i++)
        {
            f>>a>>b>>pondere;
            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.back();
    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 (vf[rad][i] == y)
                return ;
        }
    }
    }
}
int main()
{
    citeste();
    bf(x);
    cout<<sum[y];
    return 0;
}