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