#include <bits/stdc++.h>
using namespace std;

vector <pair<int,int> >h[30005];
int n,m,x,y;
int viz[30005];

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

int bfs(int k)
{
    queue <pair <int, int> >q;
    int dist;
    q.push({k,0});
    viz[k]=1;
    while(!q.empty())
    {
        k=q.front().first;
        dist=q.front().second;
        q.pop();
        for(auto e:h[k])
        {
            if(viz[e.first]==0)
            {
                viz[e.first]=1;
                q.push({e.first, dist+e.second});
                if(e.first==y)
                    return dist+e.second;
            }
        }
    }
    return 0;
}

int main()
{
    int i,j,k,dist;
    f>>n>>m>>x>>y;
    for(k=1;k<=m;k++)
    {
        f>>i>>j>>dist;
        if(i>j) swap(i,j);
        h[i].push_back({j,dist});
        h[j].push_back({i,-dist});
    }
    g<<bfs(x)<<'\n';
    return 0;
}