#include <bits/stdc++.h>
using namespace std;
const int inf = 1 << 30;
const long long linf = 1LL << 62;
const int mod = 1e9 + 7;
const int MAX = 510000;
int main() {
  long long n;
  cin >> n;
  vector<long long> a(n);
  for (long long i = 0; i < (n); i++) cin >> a[i];
  long long ans = 0;
  for (long long i = 0; i < (n); i++) {
    if (a[i] == i + 1) {
      swap(a[i], a[i + 1]);
      ans++;
    }
  }
  cout << ans << endl;
}
