codeforces #371div2 B

    #include <bits/stdc++.h>  
    #define INF 0x3f3f3f3f  
    #define N 100000+5  
    typedef long long ll;  
    using namespace std;  
      
    int a[N];  
    int main()  
    {  
        ios_base::sync_with_stdio(0);  
        cin.tie(0);  
      
        int n;  
        while(cin>>n){  
            int ok = 1;  
            for(int i = 1;i <= n;++ i)  
                cin>>a[i];  
      
            sort(a+1,a+n+1);  
            int len = unique(a+1,a+n+1)-a;  
            len--;  
            if(len > 3)  
                ok = 0;  
            else{  
                if(len == 3){  
                    if(abs(a[1]-a[2]) != abs(a[2]-a[3]))  
                        ok = 0;  
                }  
            }  
            if(ok)  cout<<"YES"<<endl;  
            else  
                cout<<"NO"<<endl;  
        }  
        return 0;  
    }