#include <bits/stdc++.h>
using namespace std;
int main() {
  ios ::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; ++i) cin >> a[i];
  vector<int> ps;
  for (int i = 0; i < n; ++i)
    if (a[i] == i + 1) ps.push_back(i);
  int ans = ps.size();
  for (int i = 1; i < ps.size(); ++i) {
    if (ps[i] == ps[i - 1] + 1) --ans;
  }
  cout << ans << endl;
  return 0;
}
