#include <iostream>
#include <fstream>
#include <vector>
#include <deque>
using namespace std;
const int NR=30005;
vector < int > v[NR];
vector < int > w[NR];
deque  < int > q;
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,x,y;
long long drum[NR];
void citire (){
    f>>n>>m>>x>>y;
    for(int i=0;i<m;i++){ //de ce sa citesti m cand poti citi n :)
        int a,b,c;
        f>>a>>b>>c;
        v[a].push_back(b);
        v[b].push_back(a);
        w[a].push_back(c);
        w[b].push_back(c);}}
void BFS (int x){
    drum[x]=1;
    q.push_back(x);
    while (!q.empty()){
        int node=q.front();
        q.pop_front();
        for(unsigned int i=0;i<v[node].size();i++)
            if(!drum[v[node][i]])
            {q.push_back(v[node][i]);
            if(v[node][i]>node) drum[v[node][i]]=drum[node]+w[node][i];
            else                drum[v[node][i]]=drum[node]-w[node][i];
            if(v[node][i]==y) {g<<drum[y]-1; return;}}}}
int main ()
{   citire();
    BFS(x);}