Codeforces Round #371 (Div. 2), problem: (B) Filya and Homework

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

#include<bits/stdc++.h>
using namespace std;
#define     pb                  push_back
#define     pii                 pair<int,int>
#define     pli                 pair<long long int,int>
#define     pbi                 pair<bool,int>
#define     pob                 pop_back
#define     all(a)              a.begin(),a.end()
#define     fio                 ios_base::sync_with_stdio(false); cin.tie(NULL);cout.tie(NULL);
#define     i64                 long long int
#define     mem(x,y)            memset(x,y,sizeof(x))
#define     fill(arr,b)         fill(arr, arr+sizeof(arr)/sizeof(arr[0]), b)
#define     maxarr(n,arr,m)      m=-1e9;for(int i=0;i<n;i++)if(arr[i]>m) m=arr[i];
#define     read                freopen("input.txt","r",stdin);
#define     sortd(x,y)          sort(x,y,greater<int>());
#define     F                   first
#define     S                   second

const int maxn = (i64)1e9+7;
const double pi = acos(-1.0);

//bool fne(const pii &a, const pii &b){return (a.F > b.F)||(a.F==b.F&&a.S>b.S);}
//bool sne(const pii &a, const pii &b){return (a.S < b.S)||(a.S==b.S&&a.F<b.F);}
//bool isprime(int n){for(int i=2;i*i<=n;i++){if(n%i==0)return false;}return true;}


int main(){
    fio;
    int n,a;
    cin>>n;
    set<int>st;
    for(int i=0;i<n;i++){
        cin>>a;
        st.insert(a);
    }
    //for(auto it:st) cout<<it<<" ";
    if(st.size()<=2)cout<<"YES";
    else if(st.size()>=4)cout<<"NO";
    else {
        auto it=st.begin();
        int x=*it;it++;
        int y=*it;it++;
        int z=*it;
        if(z-y==y-x) cout<<"YES";
        else cout<<"NO";
    }
    return 0;
}