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