#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int S = 0;
  int seq = 0;
  int p;
  for (int i = 1; i <= N; i++) {
    cin >> p;
    if (i != p) {
      S += seq / 2 + seq % 2;
      seq = 0;
    }
    if (i == p) seq++;
  }
  cout << S << endl;
  return 0;
}
