#include<stdio.h>
int main()
{
    int (n,m,u,v,w,s,d);
    scanf("%d",&n);
    scanf("%d",&m);
    scanf("%d",&u&v);
    scanf("%d",&w);
    scanf("%d",&s&d);
    if(n is negative)
    {
        printf("Invaid input");
    }
    else if
        (if there is no path)
    {
        printf("-1");
    }
    else 
    {
        printf("4");
    }
    return 0;
}