#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], p[i]--;
  int ans = 0;
  for (int i = 0; i < N - 1; i++)
    if (p[i] == i) swap(p[i], p[i + 1]), ans++;
  cout << ans << endl;
  return 0;
}
