#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int N;
  cin >> N;
  vector<int> p(N);
  for (int i = 0; i < N; i++) cin >> p[i];
  int ans = 0;
  for (int i = 0; i <= N - 1; i++) {
    if (p[i] == i + 1 && p[i + 1] == i + 2) {
      ans++;
      i++;
    } else if (p[i + 1] == i + 2 && p[i] != i + 2) {
      ans++;
      i++;
    } else if (p[i] == i + 1)
      ans++;
  }
  cout << ans << endl;
}
