def b_search(n,lst):
    start = 0
    end = len(lst) - 1
    while start < end:
        middle = (start + end) // 2
        if n > lst[middle]:
            start = middle + 1
        elif n < lst[middle]:
            end = middle - 1
        else:
            return lst[middle]
    return False

print(b_search(100,[1,2,3,34,56,57,78,87]))

def max_common(a,b):
    if a < b:
        a, b = b, a
    while b:
        t = a % b
        a = b
        b = t
    return a