#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  bool status = true, allsame = true;
  int ep = n - 1, sp = 0, count = 0;
  while (ep > 0 && a[ep] <= a[sp]) {
    ep--;
    sp--;
    if (sp < 0) sp = n - 1;
    if (a[ep] != a[sp]) allsame = false;
    count++;
  }
  int next = (sp == n - 1) ? 0 : sp + 1;
  while (sp != ep) {
    if (a[next] < a[sp]) {
      status = false;
      cout << -1 << endl;
      break;
    }
    sp++;
    sp = sp % n;
    next = (sp == n - 1) ? 0 : sp + 1;
  }
  if (status && !allsame) cout << count << endl;
  if (allsame && status) cout << 0 << endl;
  return 0;
}
