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