One day, Twilight Sparkle is interested in how to sort a sequence of integers a1, a2, ..., an in non-decreasing order. Being a young unicorn, the only operation she can perform is a unit shift. That is, she can move the last element of the sequence to its beginning:

a1, a2, ..., an → an, a1, a2, ..., an - 1.

Help Twilight Sparkle to calculate: what is the minimum number of operations that she needs to sort the sequence?

Input

The first line contains an integer n (2 ≤ n ≤ 105). The second line contains n integer numbers a1, a2, ..., an (1 ≤ ai ≤ 105).

Output

If it's impossible to sort the sequence output -1. Otherwise output the minimum number of operations Twilight Sparkle needs to sort it.

Examples

Input

2
2 1


Output

1


Input

3
1 3 2


Output

-1


Input

2
1 2


Output

0
#include <bits/stdc++.h>
using namespace std;
const int inf = 2000000000;
static inline int Rint() {
  struct X {
    int dig[256];
    X() {
      for (int i = '0'; i <= '9'; ++i) dig[i] = 1;
      dig['-'] = 1;
    }
  };
  static X fuck;
  int s = 1, v = 0, c;
  for (; !fuck.dig[c = getchar()];)
    ;
  if (c == '-')
    s = 0;
  else if (fuck.dig[c])
    v = c ^ 48;
  for (; fuck.dig[c = getchar()]; v = v * 10 + (c ^ 48))
    ;
  return s ? v : -v;
}
template <typename T>
static inline void cmax(T& a, const T& b) {
  if (b > a) a = b;
}
template <typename T>
static inline void cmin(T& a, const T& b) {
  if (b < a) a = b;
}
const int maxn = 100005 * 2;
int data[maxn];
int dp[maxn];
int main() {
  const int n = Rint();
  for (int i = 1; i <= n; ++i) data[i] = data[i + n] = Rint();
  dp[2 * n] = 1;
  for (int i = 2 * n - 1; i >= 1; --i) {
    if (data[i] <= data[i + 1])
      dp[i] = dp[i + 1] + 1;
    else
      dp[i] = 1;
  }
  int ans = inf;
  for (int i = 1; i <= n; ++i)
    if (dp[i] >= n) {
      int t = 0;
      if (i > 1) {
        t = n - i + 1;
      }
      if (t < ans) ans = t;
    }
  if (ans == inf)
    puts("-1");
  else
    printf("%d\n", ans);
  return 0;
}
