#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 i = 0; i < (n - 1); i++) {
    if (a[i] == i + 1) {
      ans++;
      int k = a[i];
      int l = a[i + 1];
      a[i] = l;
      a[i + 1] = k;
    }
  }
  cout << ans << endl;
}
