"""
You need to verify if the given credit card number is valid. For that you need to use the Luhn test. 
 
Here is the Luhn formula: 
1. Reverse the number. 
2. Multiple every second digit by 2.  
3. Subtract 9 from all numbers higher than 9. 
4. Add all the digits together. 
5. Modulo 10 of that sum should be equal to 0.  
 
Task:  
Given a credit card number, validate that it is valid using the Luhn test. Also, all valid cards must have exactly 16 digits. 
 
Input Format: 
A string containing the credit card number you need to verify. 
 
Output Format: 
A string: 'valid' in case the input is a valid credit card number (passes the Luhn test and is 16 digits long), or 'not valid', 
if it's not. 
 
Sample Input: 
4091131560563988 
 
Sample Output: 
valid
"""

nums=input()[::-1]
nums2=[]
for i in range(len(nums)):
    if i%2!=0:
        nums2.append(int(nums[i])*2)
    else:
        nums2.append(int(nums[i]))
nums3=[]        
for i in nums2:
    if i>9:
        nums3.append(i-9)
    else:
        nums3.append(i)
if sum(nums3)%10==0 and len(nums3)==16:
    print("valid")
else:
    print('not valid')
