#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; ++i) cin >> a[i], --a[i];
  int ans = 0;
  for (int i = 0; i < n - 1; ++i) {
    if (i != a[i]) continue;
    swap(a[i], a[i + 1]);
    ++ans;
  }
  cout << ans << endl;
}
