#include <bits/stdc++.h>
using namespace std;
const int MAX = 200005;
const int INF = 1001001001;
const int MOD = 1000000007;
int main() {
  int N;
  cin >> N;
  vector<int> p(N);
  for (int i = 0; i < (N); ++i) {
    cin >> p[i];
    p[i]--;
  }
  int ans = 0;
  for (int i = 0; i < (N - 1); ++i) {
    if (p[i] == i) {
      swap(p[i], p[i + 1]);
      ans++;
    }
  }
  cout << ans << endl;
}
