95B - Lucky Numbers - CodeForces Solution

def nr47(a):
	n4 = a.count('4')
	n7 = a.count('7')
	pre = 0
	if n4 < n7:
		need = (n7 - n4) // 2
		i = 1
		k = 0
		while k < need + 1 or (a[-i]) == '7':
		    k += (a[-i] == '7')
		    i += 1
		a = a[:-i] + '7'
		a += '4' * (need + i - k)
		a += '7' * (k - need - 1)
	elif n4 > n7:
		need = (n4 - n7) // 2
		i = 1
		k = 0
		while k < need:
			k += a[-i] == '4'
			i += 1
		i -= 1
		a = a[:-i] + '7' * i
	return a
def n47(a):
    ret = ''
    g = len(a)
    i = 0
    for t in a:
        if int(t) < 4:
            ret += (g - i) * '4'
            break
        elif int(t) == 4:
            ret += '4'
            n4 = i
        elif int(t) < 7:
            ret += '7' + '4' * (g - i - 1)
            break
        elif int(t) == 7:
            ret += '7'
        else:
            ret = ret[:n4] + '7' + '4' * (g - n4 - 1)
            break
        i += 1
    return ret
def big(a):
	global k
	o = 1
	for t in a:
		if int(t) > 7 or (o > k // 2 and int(t) > 4):
			return True
		elif (int(t) < 7 and not (o > k // 2)) or (int(t) < 4 and (o > k // 2)):
			return False
		o += 1
	return False
a = input()
k = len(a)
if k % 2 == 1:
	print('4' * ((k + 1) // 2) + '7' * ((k + 1) // 2))
else:
	if big(a):
		print('4' * (k // 2 + 1) + '7' * (k // 2 + 1))
	else:
	    a = nr47(n47(a))
	    print(a)