//#include <iostream>
#include <cstdio>
#define in "sate.in","r",stdin
#define out "sate.out","w",stdout
#include <vector>
#define nmax 30005
using namespace std;
int n,m,X,Y,sol[nmax],P[nmax];
vector <pair <int,int> >G[nmax];
inline void Create(){
    for(int i = 0; i < nmax; i++)
        sol[i] = -1;
}
inline void bfs(){
    int i,j,ok,k;
    k = 1;
  P[1] = X;
  sol[X] = ok = 0;
 for(i = 1; i <= k && !ok ;i++)
    for(j = 0 ; j < G[P[i]].size() && !ok;j++)
        if(sol[G[P[i]][j].first] == -1){
            P[++k] = G[P[i]][j].first;
            sol[G[P[i]][j].first] = sol[P[i]] + G[P[i]][j].second;
            if(G[P[i]][j].first == Y) ok = 1;
        }
}
int main(){
    int i,j,c;
 freopen(in);
 freopen(out);
 scanf("%d%d%d%d",&n,&m,&X,&Y);
 while(m--){
    scanf("%d%d%d",&i,&j,&c);
    G[i].push_back(make_pair(j,c));
    G[j].push_back(make_pair(i,-c));
 }
 Create();
 bfs();
 printf("%d\n",sol[Y]);
return 0;
}