#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int p[n];
  for (int i = 0; i < n; i++) cin >> p[i];
  int ans = 0;
  int tmp = 0;
  if (n % 2 == 0) {
    for (int i = 0; i < n - 1; i += 2)
      if (p[i] == i + 1 || p[i + 1] == i + 2) ans++;
    if (p[0] != 1 && p[n - 1] != n)
      for (int i = 1; i < n - 1; i += 2)
        if (p[i] == i + 1 || p[i + 1] == i + 2) tmp++;
    ans = min(ans, tmp);
  } else {
    if (p[n - 1] != n)
      for (int i = 0; i < n - 2; i += 2)
        if (p[i] == i + 1 || p[i + 1] == i + 2) ans++;
    if (p[0] != 1)
      for (int i = 1; i < n - 1; i += 2)
        if (p[i] == i + 1 || p[i + 1] == i + 2) tmp++;
    ans = min(ans, tmp);
  }
  cout << ans;
}
