#include <fstream>
#include <algorithm>

using namespace std;

ifstream cin ("secv.in");
ofstream cout ("secv.out");

const int nmax = 5000;

int n, dist;
int lim, lg, pz, pz2;

int v[1 + nmax], temp[1 + nmax];
int l[1 + nmax], p[1 + nmax];

int search(int x) {
  int st = 0, dr = lim, mid;
  while(st <= dr) {
    mid = (st + dr) / 2;
    if(v[l[mid]] < x && x <= v[l[mid + 1]])
      return mid;
    else if(v[l[mid + 1]] < x)
      st = mid + 1;
    else
      dr = mid - 1;
  }
  return lim;
}

int main() {
  cin >> n;
  for(int i = 1; i <= n; i++) {
    cin >> v[i];
    temp[i] = v[i];
  }
  sort(temp + 1, temp + n + 1);
  dist = 1;
  for(int i = 2; i <= n; i++)
    if(temp[i - 1] != temp[i])
      dist++;
  l[1] = lim = 1;
  for(int i = 2; i <= n; i++) {
    int poz = search(v[i]);
    l[poz + 1] = i;
    p[i] = l[poz];
    lim = max(lim, poz + 1);
    if(poz + 1 > lg) {
      lg = poz + 1;
      pz = i;
    }
  }
  if(lg != dist)
    cout << -1;
  else {
    pz2 = pz;
    while(p[pz])
      pz = p[pz];
    cout << pz2 - pz + 1;
  }
  return 0;
}