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