#include <fstream>
#define mx LONG_LONG_MAX
#define nmax 30050
#include <queue>
#include <vector>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,x,y,i,a,b,nod;
long long dist,dis,di,d[nmax];
vector <pair <int,long long> >v[nmax];
queue <pair<int,long long> > q;
bool ok;
int main()
{
    f>>n>>m>>x>>y;
    for(i=1; i<=m; ++i)
    {
        f>>a>>b>>dis;
        v[a].push_back({b,dis});
        v[b].push_back({a,-dis});
    }
    q.push({x,0});
    while(!q.empty()&&!ok)
    {
        nod=q.front().first;
        di=q.front().second;
        q.pop();
        for(i=0; i<v[nod].size(); ++i)
        {
            if(d[v[nod][i].first]==0&&v[nod][i].first!=x)
            {
                d[v[nod][i].first]=di+v[nod][i].second;
                q.push({v[nod][i].first,d[v[nod][i].first]});

            }
        }
        if(d[y]!=0) ok=1;
    }
    g<<d[y];

    return 0;
}