#include <bits/stdc++.h>
using namespace std;
int a[100001];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  int cnt = 0;
  for (int i = 1; i <= n - 1; i++) {
    if (a[i] == i) {
      int tmp = a[i + 1];
      a[i + 1] = a[i];
      a[i] = tmp;
      cnt += 1;
    }
  }
  cout << cnt << endl;
  return 0;
}
