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