#include<fstream>
#include<vector>
using namespace std;
int n,m,i,x,y,fin,j,s,k;
vector <int> l[30002];
int c[100003];
int v[30002];
vector <int> dis[30002];

void read()
{
    ifstream f("sate.in");
    f>>n>>m>>s>>fin;
    for(int i=1;i<=m;i++)
    {
        f>>x>>y>>k;
        l[x].push_back(y);
        dis[x].push_back(k);
        l[y].push_back(x);
        dis[y].push_back(k);
    }
    f.close();
}

int bfs(int nod)
{
    int st,dr;
    st=1;
    dr=1;
    v[nod]=1;
    c[dr]=nod;
    while(st<=dr)
    {
       k=c[st];
       for(int i=0;i<l[k].size();i++)
       {
           if(v[l[k][i]]==0)
           {
           if(l[k][i]>c[st])
           {
             v[l[k][i]]=v[k]+dis[k][i];
             c[++dr]=l[k][i];
           }
           else
           {
             v[l[k][i]]=v[k]-dis[k][i];
             c[++dr]=l[k][i];
           }
           }
       }
       st++;
       if(c[st-1]==fin) st=dr+1;
    }
}

void write()
{
    ofstream g("sate.out");
    g<<v[fin]-1;
    g.close();
}

int main()
{
    read();
    bfs(s);
    write();
    return 0;
}