#include <cstdio>
#include <vector>

using namespace std;

vector < pair < int, int > > v[100205];
int viz[30005],dist[30005];
int myabs(int b)
{
    if(b<0)
       b=-b;
    return b;
}
void dfs(int nod)
{
    viz[nod]=1;
    for(int i=0;i<v[nod].size();i++)
    {
        if(viz[v[nod][i].first]==0)
        {
            dist[v[nod][i].first]=dist[nod]+v[nod][i].second;
            dfs(v[nod][i].first);
        }
    }
}

int main()
{

    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    int n,m,x,y,a,b,d;
    scanf("%d%d%d%d",&n,&m,&x,&y);
    for(int i=1; i<=m; i++)
    {
        scanf("%d%d%d",&a,&b,&d);
        v[a].push_back({b,d});
        v[b].push_back({a,-d});
    }
    viz[x]=1;
    dfs(x);
    printf("%d",myabs(dist[y]));
    return 0;
}