#include <fstream>
#include <vector>
using namespace std;
ifstream d("sate.in");
ofstream o("sate.out");
int i,j,k,m,n,t,v[30005],x,y,b,c,cost,costf;
vector <int> a[30005],pc,co[30005];
vector <int>::iterator it,itc;
int main()
  {
    d>>n>>m>>x>>y;
    for (i=1;i<=m;i++)
      {
        d>>b>>c;
        a[b].push_back(c);
        a[c].push_back(b);
        d>>cost;
        co[b].push_back(cost);
        co[c].push_back(cost);
      };
    for (i=1;i<=n;i++) v[i]=-1;
    pc.push_back(x);
    v[x]=0;
    while (!pc.empty())
      {
        b=pc.back();
        pc.pop_back();
        for (it=a[b].begin(), itc=co[b].begin(); it!=a[b].end(); ++it, ++itc)
          if (v[*it]==-1)
            {
              if (b<*it) v[*it]=v[b]+*itc;
                else v[*it]=v[b]-*itc;
              pc.push_back(*it);
            };
      };
    o<<v[y];
  }