#include <fstream>
#include<queue>
#include<algorithm>
#define valoare 99999999
using namespace std;
int main()
{
int *dist,k,i,x,y,n,m,b,c,d;
vector< pair <int,int> > *v;
queue<int> q;
ifstream f("sate.in");
ofstream g("sate.out");
f >> n >> m >> b >>c;
v = new vector< pair< int,int > >[n+1];
dist = new int [n+1];
for(i=0;i<m;i++)
    {
    f >> x >> y >> d;
    v[x].push_back(make_pair(y,d));
    v[y].push_back(make_pair(x,-d));
    }
for(i=1;i<=n;i++)
    {
    dist[i]=valoare;
    }
    dist[b]=0;
q.push(b);
while(!q.empty())
{
k=q.front();
q.pop();
for(vector< pair<int,int > >::iterator i=v[k].begin();i!=v[k].end();i++)
    {
    if(dist[i->first]==valoare)
        {
        dist[i->first]=dist[k]+i->second;
        }
    if(i->first==c)
        {
        g<<dist[i->first];
        return 0;
        }
    q.push(i->first);
    }
}
f.close();
g.close();
return 0;
}