#include <iostream>
#include <vector>
#include <fstream>
#include <queue>

using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

long long n,m,x,y,viz[30001],d[30001];

vector <pair <int,int> > a[30001];
queue  <int> q;

void citire()
{
    int i,j,k,c;
    f>>n>>m>>x>>y;
    for(k=0;k<m;k++)
    {
        f>>i>>j>>c;
        a[i].push_back({j,c});
        a[j].push_back({i,c});
    }
}

int bfs(int i)
{
    int p,j,c,v;
    viz[i]=0;
    q.push(i);
    while(q.size()!=0)
    {
        p=q.front();
        for(j=0;j<a[p].size();j++)
        {
            v=a[p][j].first;
            c=a[p][j].second;
            if(viz[v]==0)
            {
                q.push(v);
                viz[v]=1;
                if(p>v)
                    d[v]=d[p]-c;
                else
                    d[v]=d[p]+c;
            }
            if(viz[y]==1)
                break;
        }
        q.pop();
        if(viz[y]==1)
            break;
    }
}

int main()
{
    citire();
    bfs(x);
    g<<d[y];
    return 0;
}