#include <fstream>
#include <vector>
#define Nmax 100004
#define In "sate.in"
#define Out "sate.out"
using namespace std;
vector< pair<short,int> >next[Nmax];
short n,pp,ps,ok = 1;
long long sum[Nmax];
bool viz[Nmax];
void Citire()
{
    ifstream f(In);
    int m,x,y,c;
    f>>n>>m>>pp>>ps;
    for(;m;m--)
    {
        f>>x>>y>>c;
        next[x].push_back(make_pair(y,c));
        next[y].push_back(make_pair(x,-c));
    }
}

inline void Dfs(short nod)
{
    viz[nod] = true;
    if(nod==ps)
    {
        return ;
        ok = 0;
    }
    for(vector< pair<short ,int> >::iterator it=next[nod].begin();ok &&it!=next[nod].end();it++)
        if(viz[(*it).first]==false)
        {
            sum[(*it).first] = sum[nod]+(*it).second;
            Dfs((*it).first);
        }
}

int main()
{
    Citire();
    Dfs(pp);
    ofstream g(Out);
    g<<sum[ps]<<"\n";
    g.close();
    return 0;
}