#include <fstream>
#include <vector>
#include <queue>
#include <string.h>
#define maxn 30005
#define INF 200000000
#define f first
#define s second

using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

int n, m, x, y, a, b, c;
int drum[maxn];
vector<pair <int, int> > G[maxn];
queue<int> Q;

void bfs(int nod)
{
    memset(drum, 0, sizeof(drum));
    Q.push(nod);
    drum[nod]=0;

    while(!Q.empty())
    {
        int p=Q.front();
        Q.pop();
        for(int i=0; i<G[p].size(); i++)
        {
            if(G[p][i].f>p)
            {
                drum[G[p][i].f]=drum[p]+G[p][i].s;
            }
            else
            {
                drum[G[p][i].f]=drum[p]-G[p][i].s;
            }
            Q.push(G[p][i].f);
            if(G[p][i].f==y)
            {
                return;
            }
        }
    }
}

int main()
{
    fin >> n >> m >> x >> y;

    for(int i=1; i<=m; i++)
    {
        fin >> a >> b >> c;
        G[a].push_back(make_pair(b, c));
        G[b].push_back(make_pair(a, c));
    }

    bfs(x);

    fout << drum[y];

    return 0;
}