#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 count = 0;
  for (int j = 0; j < n - 1; j++) {
    if (a[j] == j + 1) {
      swap(a[j], a[j + 1]);
      count++;
    }
  }
  cout << count << endl;
  return 0;
}
