#include <iostream>
using namespace std;
int main(){
  int n;
  cin >> n;
  int a[n];
  for(int i = 0; i < n; i++)cin >> a[i];
  int count = 0;
  for(int j = 0; j < n; j++){
    if(a[j] == j+1){
      swap(a[j], a[j+1]);
      count++;
    }
  }
  
  cout << count << endl;
  return 0;
}