Level1 없는 숫자 더하기

def solution(numbers):
    answer = -1
    
    numbers.sort()
    n = len(numbers)
    
    sum=0; k=0
    
    for i in range(0,10):
        if(k<n):
            if(numbers[k]==i):k+=1
            else: sum+=i    
        else: sum+=i 
        
    return sum