# include <cstdio>
# include <queue>
# include <vector>

# define nmax 30005

using namespace std;

queue <int> Q;
vector <pair <int,int> > G[nmax];
int n, m, a, b, c, x, y, nod;
int viz[nmax], dist[nmax];

int main()
{int i;
    
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    
    scanf("%d%d%d%d",&n,&m,&x,&y);
    for (i = 1; i <= m; i++)
    {
        scanf("%d%d%d",&a,&b,&c);
        if (a > b) swap(a,b);
        G[a].push_back(make_pair(b,c));
        G[b].push_back(make_pair(a,-c));
        }
        
    Q.push(x);
    viz[x] = 1;
    for ( ; !Q.empty(); )
    {
        nod = Q.front();
        Q.pop();
        for (i = 0; i < G[nod].size(); i++)
            if (!viz[G[nod][i].first])
            {
                  viz[G[nod][i].first] = 1;
                  dist[G[nod][i].first] += dist[nod] + G[nod][i].second;
                  Q.push(G[nod][i].first);
                  }
        }
        
    printf("%d",dist[y]);
    
return 0;
}