#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n, a[100100], cnt = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  for (int i = 0; i < n - 1; ++i) {
    if (a[i] == (i + 1)) {
      swap(a[i], a[i + 1]);
      ++cnt;
    }
  }
  cout << cnt << endl;
}
