cf 714B

#include<bits\stdc++.h>

using namespace std;

int main()
{
    int n, i;
    scanf("%d", &n);

    long long a[n];
    for(i = 0; i < n; i++) scanf("%lld", &a[i]);

    sort(a, a + n);
    int cnt = 1;
    for(i = 0; i < n - 1; i++)
    {
        if(a[i] != a[i + 1])
        {
            cnt++;
        }
    }

    //printf("cnt %d %lld %lld %lld\n", cnt, *max_element(a, a + n), mid, *min_element(a, a + n));
    if(cnt == 3)
    {
        long long mid;
        for(i = 0; i < n - 1; i++)
        {
            if(a[i] != a[i + 1])
            {
                mid = a[i+1];
                break;
            }
        }
        if(2*mid == *max_element(a, a + n) + *min_element(a, a + n)) printf("YES\n");
        else printf("NO\n");
    }
    else if(cnt == 1 || cnt == 2) printf("YES\n");
    else printf("NO\n");
}