#include<fstream>
using namespace std;
ifstream in("secv.in");
ofstream out("secv.out");

long long v[5009];
int N;

int main()
{

    in>>N;
    int i,poz;
    long long minim = 20000000000,maxim = -1;
    for(i = 1 ; i <= N ; i++)
    {
        in>>v[i];
        minim = min(v[i],minim);
        maxim = max(v[i],maxim);
    }
    i = N;
    while(v[i] != maxim) i--;
    poz = i;
    bool b;
    for(int j = maxim - 1 ; j >= 1 ; j--)
    {
        b = false;
        while(v[i] != j){
            i--;
            if(i == 0) {
                b = true;
                break;
            }
        }
        if(b) break;
    }
    if(b) out<<-1;
    else out<<poz-i+1;
    out.close();
    in.close();
    return 0;
}