#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e6;
const long long MOD = 1e9 + 7;
const long long N = 1e6 + 7;
const long long N_N = 1e3;
const int INF = 1e9 + 228;
const long double eps = 1e-12;
long long n, a[N], ans, l, ans1;
int main() {
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    cin >> a[i];
  }
  for (int i = 2; i <= n; ++i) {
    if (a[i - 1] > a[i]) {
      ans++;
      ans1 = i;
    }
  }
  if (ans > 1) {
    cout << -1;
  }
  if (ans == 0) {
    cout << 0;
  }
  if (ans == 1) {
    if (a[1] >= a[ans1] && a[1] >= a[n]) {
      cout << n - ans1 + 1;
    } else
      cout << -1;
  }
  return 0;
}
