#include<iostream>
#include<cstdio>
#include<vector>

using namespace std;

int viz[30001], d[30001];
vector<pair<int, int> >g[30001];
void DF(int nod)
{
    int i;
    for(i=0; i<g[nod].size(); i++)
    {
        if(viz[g[nod][i].first]==0)
        {
            viz[g[nod][i].first]=1;
            if(g[nod][i].first<nod)
                d[g[nod][i].first]=d[nod]-g[nod][i].second;
            if(g[nod][i].first>nod)
                d[g[nod][i].first]=d[nod]+g[nod][i].second;
            DF(g[nod][i].first);
        }

    }

}
int main()
{
    FILE *fin, *fout;
    fin=fopen("sate.in", "r");
    fout=fopen("sate.out", "w");

    int n, m, x, y, i, a, b, c;
    fscanf(fin, "%d %d %d %d", &n, &m, &x, &y);
    for(i=0; i<m; i++)
    {
        fscanf(fin, "%d %d %d", &a, &b, &c);
        g[a].push_back(make_pair(b, c));
        g[b].push_back(make_pair(a, c));
    }
    d[x]=0;
    viz[x]=1;
    DF(x);
    fprintf(fout, "%d", d[y]);
}