find missing number from a list range

 l1=[1,2,5,6]

l2=[7, 1, 3, 5, 6]

def find_missing_number(l1):

    max_n=max(l1)+1

    min_n=min(l1) 

    c=[a for a in range(min_n,max_n) ]

    missing_n=[a for a in c if a not in l1]

    return missing_n

print(find_missing_number(l1))

print(find_missing_number(l2))

[3, 4]

[2, 4]