"""
Exercise 2: Add Binary (LeetCode 67)

Return the sum of two binary strings as a binary string. Walk both strings
from the end with a carry (practise the algorithm; don't use int(a, 2)).
"""

def add_binary(a, b):
    # TODO
    return ""


# --- self-check: do not modify below this line ---
x, y, z = add_binary("11", "1"), add_binary("1010", "1011"), add_binary("1" * 80, "1")
print(x, y, len(z))
print("PASS" if x == "100" and y == "10101" and z == "1" + "0" * 80 else "FAIL")