#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int> p(N);
  for (int i = 0; i < N; ++i) cin >> p[i];
  int ans = 0;
  for (int i = 0; i < N - 1; ++i) {
    int tmp;
    if (p[i] == (i + 1) || p[i + 1] == (i + 2)) {
      ans++;
      tmp = p[i];
      p[i] = p[i + 1];
      p[i + 1] = tmp;
    }
  }
  cout << ans << endl;
  return 0;
}
