#include <bits/stdc++.h>
using namespace std;


int n;
int red[50000];
bool taken[100001];

int main() {
    ifstream cin("highcard.in");
    ofstream cout("highcard.out");

    cin >> n;

    for (int i=0; i<n; i++) {
        cin >> red[i];
        taken[red[i]] = true;
    }

    sort(red, red + n);

    vector<int> p1;

  for(int i=1; i<=n*2; i++){
    if (!taken[i]) {
      p1.push_back(i);
    }
  }

  sort(p1.begin(), p1.end());


  int maxP = 0;
  int curI = 0;
  for(int i =0; i<n; i++){
    for(int j=curI; j<n; j++){
      if (p1[j] > red[i]){
        maxP += 1;
        curI = j;
        break;
      }
    }
  }

    cout << maxP << endl;
}