#include <fstream>
#include <queue>
#include <vector>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
vector<pair<int,int> > vec[30003];
bool ok[30003];
queue<pair<int,int> > q;
int main()
{
    int n,m,x,y,a,b,cost;
    cin>>n>>m>>x>>y;
    for(int i=1;i<=m;++i)
    {
        cin>>a>>b>>cost;
        vec[a].push_back({b,cost});
        vec[b].push_back({a,-cost});
    }
    q.push({x,0});
    ok[x]=1;
    while(q.front().first!=y)
    {
        int nod=q.front().first;
        int dist=q.front().second;
        ok[nod]=1;
        q.pop();
        for(int i=0;i<vec[nod].size();++i)
        if(ok[vec[nod][i].first]==0)
        ok[vec[nod][i].first]=1,q.push({vec[nod][i].first,dist+vec[nod][i].second});
    }
    cout<<q.front().second;
    return 0;
}