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