#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> p(n, 0);
  for (int i = 0; i < n; i++) {
    cin >> p[i];
  }
  int counter = 0;
  for (int i = 0; i < n - 1; i++) {
    if (p[i] == i + 1) {
      counter++;
      i++;
    }
  }
  if (p[n - 1] == n && n % 2 != 0) {
    counter++;
  }
  cout << counter << endl;
}
