#include <stdio.h>
int abs(int x){
        if(x<0) return -x;
        return x;
}
int N;
int main(){
        scanf("%d", &N);
        int x[N];
        for(int i=0;i<N;i++){
                scanf("%d", &x[i]);
        }
        int count0 =0,count =0;
        for(int i=0;i<N-1;i++){
                if(abs(x[i+1]-x[i])==0){
                        count0 = 1;
                }else if(abs(x[i+1]-x[i]) == abs(x[i+2]-x[i+1])){
                        count ++;
                }
        }
        if(count0 == 1){
                printf("NO\n");
        }else if(count == N-2){
                printf("YES\n");
        }else{
                printf("NO\n");
        }
        return 0;
}