#include <bits/stdc++.h>
#include <cstring>
#define pb push_back
#define mp make_pair
#define f first
#define s second
#define INF 0x3f3f3f3f
#define NMAX 1000005
using namespace std;

int n,m,a,b,cst[NMAX],x=0,y=0,c=0;
queue<int> Q;
vector<pair<int,int> > v[NMAX];
vector<pair<int,int> >::iterator it;
void bfs()
{
    int x=0,K=0,cost=0;
    Q.push(a);
    cst[a]=1;
    if(a==b) return;
    while(!Q.empty())
    {
        x=Q.front();
        for(it=v[x].begin();it!=v[x].end();it++)
        {
            K=(*it).f;
            cost=(*it).s;
            if(!cst[K])
            {
                cst[K]=cst[x]+cost;
                if(K==b)
                    return;
                Q.push(K);
            }
        }
        Q.pop();
    }
}
int main()
{
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    scanf("%d%d%d%d",&n,&m,&a,&b);
    for(int i=1;i<=m;i++)
    {
        scanf("%d%d%d",&x,&y,&c);
        v[x].push_back(make_pair(y,c));
        v[y].push_back(make_pair(x,-c));
    }
    bfs();
    printf("%d",cst[b]-1);
    return 0;
}