#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int A[100010];
  for (int i = 1; i <= N; i++) {
    int j;
    cin >> j;
    if (i == j) {
      A[i] = A[i - 1] + 1;
    }
  }
  int ans = 0;
  for (int i = 1; i <= N; i++) {
    if (A[i + 1] == 0) {
      ans += (A[i] + 1) / 2;
    }
  }
  cout << ans << endl;
}
