#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
const int NR=30005;
vector < int > v[NR];
vector < int > w[NR];
ifstream f("sate.in");
ofstream g("sate.out");
int n,m,x,y,sw=0;
long long drum[NR];
void citire (){
    f>>n>>m>>x>>y;
    for(int i=0;i<m;i++){
        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 DFS (int x){

        for(unsigned int i=0;i<v[x].size();i++)
            if(!drum[v[x][i]])
            {
            if(v[x][i]>x) drum[v[x][i]]=drum[x]+w[x][i];
            else                drum[v[x][i]]=drum[x]-w[x][i];
            if(v[x][i]==y) {g<<drum[y]-1;sw=1; return;}
            DFS(v[x][i]);if(sw) return;}}
int main ()
{   citire();
drum[x]=1;
    DFS(x);}