#include <bits/stdc++.h>
using namespace std;
const int maxn = 100000 + 5;
int A[maxn];
deque<int> que;
int main() {
  int n;
  while (scanf("%d", &n) != EOF) {
    que.clear();
    int d;
    for (int i = 1; i <= n; ++i) {
      scanf("%d", &d);
      que.push_back(d);
    }
    if (n == 1) {
      puts("0");
      continue;
    }
    int ci = n;
    while (*(que.end() - 1) <= *que.begin() && ci > 0) {
      ci--;
      int temp = *(que.end() - 1);
      que.pop_back();
      que.push_front(temp);
    }
    int t = *que.begin(), flag = 0, tt = *que.begin();
    while (!que.empty()) {
      if (t > *que.begin()) {
        flag = -1;
        break;
      }
      t = *que.begin();
      if (t != tt) flag = 1;
      que.pop_front();
    }
    if (flag == -1 || flag == 0)
      printf("%d\n", flag);
    else
      printf("%d\n", n - ci);
  }
  return 0;
}
