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