1.求逆序数对的个数 

def inverse_num(a):
    if a==0:
        return 0
    else:
        num_list=list(str(a))
        while num_list[-1]=='0':
            num_list.pop()
        temp=''.join(num_list[::-1])
        return temp
if __name__=='__main__':
    n=int(input())
    count=0
    list1=[]
    for i in range(1,n+1):
        if i*4==int(inverse_num(i)):
            count+=1
            list1.append(i)
            list1.append(int(inverse_num(i)))
    print(count)
    for i in range(0,len(list1),2):
        print(list1[i],list1[i+1])