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