#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, ans, i;
  cin >> n;
  int sw;
  vector<int> p(n);
  for (i = 0; i < n; ++i) {
    cin >> p[i];
  }
  ans = 0;
  for (i = 0; i < n; ++i) {
    if (p[i] == (i + 1)) {
      sw = p[i];
      p[i] = p[i + 1];
      p[i + i] = sw;
      ans++;
    }
  }
  cout << ans << endl;
}
