#include <bits/stdc++.h>
using namespace std;
int a[123456];
int n;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  int cnt = 0;
  for (int i = 1; i < n; i++) {
    if ((a[i] == i && a[i + 1] != i + 1) || (a[i] == i && a[i + 1] == i + 1)) {
      cnt++;
      swap(a[i], a[i + 1]);
    }
  }
  cout << cnt;
}
