#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1001001001;
const int MOD = 1000000007;
int main() {
  int N;
  cin >> N;
  vector<int> p(N);
  for (int i = 0; i < (int)(N); i++) {
    cin >> p[i];
    p[i]--;
  }
  int cnt = 0;
  for (int i = 0; i < N - 1; i++) {
    if (i == p[i]) {
      swap(p[i], p[i + 1]);
      cnt++;
    }
  }
  cout << cnt << endl;
  return 0;
}
