keyboards.sort()
    drives.sort(reverse=True)
    
    best = -1
    i = j = 0
    
    while i < len(keyboards) and j < len(drives):
        total = keyboards[i] + drives[j]
        if total > b:
            j += 1
        else:
            if total > best:
                best = total
            i += 1
    return best