#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;
int n,m,X,Y,x,y,i,viz[30001];
long c;
vector < pair <int,long> >v[30001];
ifstream f("sate.in");
ofstream g("sate.out");
void BF(int x)
{
    int p,u,C[30001],i,k;
    queue <int> c;
    p=u=1;
    C[p]=x;
    viz[x]=1;
    while(p<=u)
    {
        k=C[p];
        for(i=0;i<v[k].size();i++)
            if(viz[v[k][i].first]==0)
        {
            if(k<v[k][i].first)
                viz[v[k][i].first]=viz[k]+v[k][i].second;
            else
                viz[v[k][i].first]=viz[k]-v[k][i].second;
            C[++u]=v[k][i].first;
        }
        p++;
    }
}
int main()
{
    f>>n>>m>>X>>Y;
    for(i=1;i<=m;i++)
    {
        f>>x>>y>>c;
        v[x].push_back({y,c});
        v[y].push_back({x,c});
    }
    BF(X);
    g<<viz[Y]-1;
    return 0;
}