int i , j , n , len , sum[ 600 ] , big ;
char s[600] , ts[7];
char *p1 , *p2;

int main(){
	scanf("%d\n", &n );
	scanf("%s" , s );
	len=strlen(s);
	memset(sum,0,sizeof(sum));
	big=-1;
	for (i=0;i+n-1<len;i++){
		if (sum[i]<0)continue;
		sum[i]=0;
		p1=&s[i];
		for (j=i+1;j+n-1<len;j++){
			p2=&s[j];
			if (strncmp(p1,p2,n)==0){
				sum[i]++;
				sum[j]=-1;
			}
		}
		if (sum[i]>big)big=sum[i];
	}
	if (big==0){
		printf("NO");
		return 0;
	}
	printf("%d\n", big+1 );
	for (i=0;i+n-1<len;i++)
		if (sum[i]==big){
			p1=&s[i];
			strncpy( ts , p1 , n );
			printf("%s\n", ts );
		}
				
	return 0;

}
