#include <fstream>
#define MAXN 100001
using namespace std;

ifstream is("cautbin.in");
ofstream os("cautbin.out");

int M,N;
int v[MAXN];

int main()
{
    int st,dr,mid,n,x,poz,cmmv;
    st = 1;
    is>>n;
    dr=n;
    for(int i=0;i<n;i++)
        is>>v[i];
    is>>x;
    while(st <= dr)
    {
        mid = (st+dr)/2;
        if( v[mid] < x )
        {
            st = mid+1;
        }
        else if( v[mid] == x)
        {
            st = mid+1;
            poz = mid;
        }
        else
            dr = mid-1;
    }
    if(x==mid)
        os<<poz;
    else
        os<<"-1";
    while(st <= dr)
    {
        mid = (st+dr)/2;
        if( v[mid] < poz )
        {
            st = mid+1;
        }
        else if( v[mid] == poz)
        {
            st = mid+1;
            cmmv = mid;
        }
        else
            dr = mid-1;
    }

    os<<cmmv;
    is.close();
    os.close();
    return 0;
}