#include <bits/stdc++.h>
using namespace std;
using namespace std;
int n, a[100005], res;
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  res = 0;
  for (int i = 1; i < n; i++)
    if (a[i] == i) {
      swap(a[i], a[i + 1]);
      res++;
    }
  cout << res << endl;
}
