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