#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int b[n];
  for (int i = 0; i < n; i++) {
    if (a[i] == i) {
      b[i] = 1;
    } else {
      b[i] = 0;
    }
  }
  for (int i = 0; i < n - 1; i++) {
    if (b[i] == 1 && b[i + 1] == 1) {
      b[i + 1] = 2;
    }
  }
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (b[i] == 1) {
      ans++;
    }
  }
  cout << ans << endl;
}
