#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);
    A.at(i)--;
  }
  int cnt = 0;
  for (int i = 0; i < N; i++) {
    if (A.at(i) == i) {
      swap(A.at(i), A.at(i + 1));
      cnt++;
    }
  }
  cout << cnt << endl;
}
