#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 cnt = 0;
  for (int i = 0; i < n - 1; i++) {
    if (a.at(i) == i + 1 || a.at(i + 1) == i + 2) {
      swap(a.at(i), a.at(i + 1));
      cnt++;
    }
  }
  cout << cnt << endl;
  return 0;
}
