DANHBAI C++

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

int n,d[100001],a[50001],b[50001];
void nhap()
{
    int u;
    scanf("%d",&n);
    for(int i = 1;i <= n;++i)
    {
        scanf("%d",&u);
        d[u]++;
    }
}
void tao_mang()
{
    int x=0,y=0;
    for(int i = 1;i <= 2*n;++i)
    {
        if(d[i]==1)
        {
            x++;a[x] = i;
        }
        else
        {
            y++;b[y] = i;
        }
    }
}
void giai()
{
    tao_mang();
    int i = 1,j = 1,t = 0;
    for(int j=1;j<=n;j++)
    {
        if(b[j] > a[i])
        {
            i++;
            t++;
        }
    }
    printf("%d",t);
}
int main()
{
    freopen("danhbai.inp","r",stdin);
    freopen("danhbai.out","w",stdout);
    nhap();
    giai();
}