[Python] Pygame screen.fill() not filling up the color properly

I've just started learning Pygame . I'm following this tutorial. I ran the following program but it shows black color instead of blue :

import pygame

h = input("Enter the height of the window : ")
w = input("Enter the width of the window : ")
screen = pygame.display.set_mode((w,h))

running = True
while running:
event = pygame.event.poll()
if event.type == pygame.QUIT:
running=0

screen.fill((0,0,1))
pygame.display.flip()

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