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