#include <bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int i,j,n,x,s,y,poz,viz[100005],nr,m,nr2,a,b,d,nod;
vector<pair<int,int>> v[30005];
queue<int> q;
int main()
{ fin>>n>>m>>x>>y;
  for(i=1;i<=m;i++)
  {fin>>a>>b>>d;
    v[a].push_back(make_pair(b,d));
    v[b].push_back(make_pair(a,d));
  }
  q.push(x);
  viz[x]=-1;
  while(q.front()!=y)
  {i=q.front();
  //cout<<i<<endl;
   for(j=0;j<v[i].size();j++)
      {nod=v[i][j].first;
       d=v[i][j].second;
       if(viz[nod]==0)
        if(nod>i) viz[nod]=viz[i]+d;
        else viz[nod]=viz[i]-d;
         q.push(nod);
      }
   q.pop();
  }
  fout<<viz[y]+1;
    return 0;
}