#include <bits/stdc++.h>
using namespace std;
int a[2234568];
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) {
      swap(a[i], a[i + 1]);
      cnt++;
    }
  }
  cout << cnt;
}
