Variable is not defined error

import random

#generate a random number
random_number = random.randint(1, 10)
print(random_number)

#Instruction of the game
print("You have to guess a number and have 3 tries")

#Ask for a number
def ask():
    guessed_number = int(input("What's your guess? "))

#innitial counter of tries 
tries = 0

while tries < 3:
     #Ask for a numbers
    ask()
    #Compare random and guess number and show if low or high 
    if guessed_number == random_number:
        print("You won")
        break
    elif guessed_number > random_number:
        print("Too high")
        tries =+1
    else:
        print("Too low")
        tries =+1