#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[i];
  }
  int ans = 0;
  for (int j = 0; j < n - 1; ++j) {
    if (a[j] == j + 1) {
      swap(a[j], a[j + 1]);
      ans++;
    }
  }
  cout << ans << endl;
}
