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