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

signed main(){
    int N; cin>>N; 
    int prev,now; cin>>prev; int tmp;
    bool ans=true;
    for(int i=1;i<N-1;i++){
        cin>>now;
        if(i!=1 and tmp!=now-prev){
            ans=false;
            break;
        }
        prev=now;
    }
    cout<<(ans ? "YES" : "NO")<<endl;
}