#include<stdio.h>
int f[1100];
bool chk[1100],r_cross[2100],l_cross[2100];
int main(){
	int n,i;
	FILE *in1 = fopen("/var/www/bbs/prob/814/input.txt","r");
	FILE *in2 = fopen("/var/www/bbs/prob/814/output.txt","r");
	fscanf(in1,"%d",&n);
	for(i=0;i<n;i++) fscanf(in2,"%d",&f[i]);
	for(i=0;i<n;i++){
		if(chk[f[i]] || r_cross[f[i]-i+1000] || l_cross[f[i]+i]) {printf("NO");return 0;}
		chk[f[i]] = 1;
		r_cross[f[i]-i+1000] = 1;
		l_cross[f[i]+i] = 1;
	}
	printf("YES");
	return 0;
}