我的做法可能有些奇怪，有没有大佬验证一下：

#include <cstdio>
#include <algorithm>
#define N 1000005
using namespace std;

int n, ans, a[N];

int main ()
{
    scanf ("%d", &n);
    for (int i = 1; i <= n; i ++)
    {
        scanf ("%d", &a[i]);
    }
    sort (a + 1, a + n + 1);
    for (int i = 1; i <= n; i ++)
    {
        ans = max (ans, i - a[i]);
    }
    printf ("%d", n - ans);
    return 0;
}