#include <fstream>
#include <vector>
#include <bitset>
using namespace std;
ifstream fin ("sate.in");
ofstream fout ("sate.out");
vector < pair < int, int> > G[30001];
bitset <30001> viz;
int dist[30001];
int n, m, x, y;
int dfs(int nod)
{
    viz[nod]=1;
    for(unsigned i=0; i<G[nod].size(); ++i)
        if(viz[G[nod][i].first]==0)
        {
            if(G[nod][i].first>nod)
                dist[G[nod][i].first]=dist[nod]+G[nod][i].second;
            else
                dist[G[nod][i].first]=dist[nod]-G[nod][i].second;
            dfs(G[nod][i].first);
        }
}
int main()
{
    fin>>n>>m>>x>>y;
    for(int i=1; i<=m; ++i)
    {
        int a, b, s;
        fin>>a>>b>>s;
        G[a].push_back(make_pair(b, s));
        G[b].push_back(make_pair(a, s));
    }
    dfs(x);
    fout<<dist[y];
    return 0;
}