#include <fstream>
#include <vector>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int n,m,x,y,a,b,c,fr[30001];
struct numar
{
    int y,cost;
};
struct coada
{
    int x,cost;
};
vector <numar> A[30001];
vector <coada> q;
int main()
{
    fin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        A[a].push_back({b,c});
        A[b].push_back({a,-c});
    }
    q.push_back({x,0});
    fr[x]=1;
    while(!q.empty())
    {
        int l=q.back().x;
        int cost=q.back().cost;
        q.pop_back();

        if(l==y)
        {
            fout<<cost;
            return 0;
        }
        for(auto i:A[l])
        {

            if(fr[i.y]==0)
            {
                fr[i.y]=1;
                 q.push_back({i.y,cost+i.cost});
            }

        }

    }

    return 0;
}