#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.at(i);
  int count = 0;
  for (int i = 0; i < N; i++) {
    if (A.at(i) == i + 1 && i < N - 1) {
      if (A.at(i + 1) == i + 2) {
        count++;
        i++;
      } else
        count++;
    }
  }
  cout << count << endl;
}
