#include <bits/stdc++.h>
using namespace std;
 
vector<pair<long long, long long> > a[31000];
bool viz[31000];
long long rez[31000], n, m, x, y;
 
void dfs(long long x, long long c)
{
    rez[x] = c;
    viz[x] = 1;
    for (long long i = 0; i < a[x].size(); i++)
        if (!viz[a[x][i].first])
            dfs(a[x][i].first, x < a[x][i].first ? c + a[x][i].second : c - a[x][i].second); 
}
 
int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    ifstream cin("sate.in");
    ofstream cout("sate.out");
    cin >> n >> m >> x >> y;
    for (long long i = 0; i < m; i++)
    {
        long long x, y, z;
        cin >> x >> y >> z;
        a[y].push_back(make_pair(x,z));
        a[x].push_back(make_pair(y,z));
    }
    dfs(x, 0);
    cout << rez[y];
    return 0;
}