#include <bits/stdc++.h>
using namespace std;
int main() {
  int a[100000], n, i, count = 0;
  cin >> n;
  for (i = 1; i <= n; i++) cin >> a[i];
  for (i = 1; i <= n; i++) {
    if (a[i] == i) {
      if (i == n)
        swap(a[i], a[i - 1]);
      else
        swap(a[i], a[i + 1]);
    }
  }
  cout << count;
}
