#include<iostream.h>
long n;
int v,a[1000000],m;
int cautare(int v)
{long p,u,s;
p=1;
u=n;
while(p<u){m=(p+u)/2;
		   if(v==a[m])return m;
           if(v>a[m])p=m+1;
		   if(v<a[m])u=m-1;
}
return 0;
}
int main()
{cout<<"n=";cin>>n;
 for(int i=1;i<=n;i++)
   {cout<<"a["<<i<<"]=";cin>>a[i];
   }
   cout<<"v=";cin>>v;
if(cautare(v))cout<<m;
else cout<<"-1";
return 0;
}