Keks solution

keks.py

def max_index(items, start, end):
    best_item = None
    best_index = None
    for index, item in zip(range(start, end), items[start:end]):
        if best_item is None or item > best_item:
            best_item = item
            best_index = index
    if best_index is None:
        raise ValueError('maximum of empty sequence')
    else:
        return best_index


def keks(num, n_skip):
    digits = list(num)
    start = 0
    for end in range(n_skip+1, len(digits)+1):
        start = max_index(digits, start, end)
        yield digits[start]
        start += 1

def main():
    with open('keks.in', 'r') as in_f:
        with open('keks.out', 'w') as out_f:
            n_digits, n_skip = map(int, in_f.readline().strip().split())
            num = in_f.readline().strip()
            print >>out_f, ''.join(keks(num, n_skip))

if __name__ == '__main__':
    main()