Buggy python submission (verdict: Runtime Error):
import math

def cmb(n, r):
    if n - r < r: r = n - r
    if r == 0: return 1
    if r == 1: return n

    numerator = [n - r + k + 1 for k in range(r)]
    denominator = [k + 1 for k in range(r)]

    for p in range(2,r+1):
        pivot = denominator[p - 1]
        if pivot > 1:
            offset = (n - r) % p
            for k in range(p-1,r,p):
                numerator[k - offset] /= pivot
                denominator[k] /= pivot

    result = 1
    for k in range(r):
        if numerator[k] > 1:
            result *= int(numerator[k])

    return result


x, y = map(int, input().split())
if ((x + y) % 3 != 0):
	print("0")
elif (x > 2*y or y > 2_x):
	print("0")
else:
	n_x = (2 * x - y) // 3
	n_y = (2 * y - x) // 3
	n_xy = (x + y) // 3
	print(cmb(n_xy,n_x)%1000000007)


Fixed submission:
import math

def cmb(n, r):
    if n - r < r: r = n - r
    if r == 0: return 1
    if r == 1: return n

    numerator = [n - r + k + 1 for k in range(r)]
    denominator = [k + 1 for k in range(r)]

    for p in range(2,r+1):
        pivot = denominator[p - 1]
        if pivot > 1:
            offset = (n - r) % p
            for k in range(p-1,r,p):
                numerator[k - offset] /= pivot
                denominator[k] /= pivot

    result = 1
    for k in range(r):
        if numerator[k] > 1:
            result *= int(numerator[k])

    return result


x, y = map(int, input().split())
if ((x + y) % 3 != 0):
	print("0")
elif (x > 2*y or y > 2*x):
	print("0")
else:
	n_x = (2 * x - y) // 3
	n_y = (2 * y - x) // 3
	n_xy = (x + y) // 3
	print(cmb(n_xy,n_x)%1000000007)

Buggy python submission (verdict: Runtime Error):
import math

def cmb(n, r):
    if n - r < r: r = n - r
    if r == 0: return 1
    if r == 1: return n

    numerator = [n - r + k + 1 for k in range(r)]
    denominator = [k + 1 for k in range(r)]

    for p in range(2,r+1):
        pivot = denominator[p - 1]
        if pivot > 1:
            offset = (n - r) % p
            for k in range(p-1,r,p):
                numerator[k - offset] /= pivot
                denominator[k] /= pivot

    result = 1
    for k in range(r):
        if numerator[k] > 1:
            result *= int(numerator[k])

    return result


x, y = map(int, input().split())
if ((x + y) % 3 != 0):
	print("0")
elif (x > 2*y || y > 2_x):
	print("0")
else:
	n_x = (2 * x - y) // 3
	n_y = (2 * y - x) // 3
	n_xy = (x + y) // 3
	print(cmb(n_xy,n_x)%1000000007)


Fixed submission:
import math

def cmb(n, r):
    if n - r < r: r = n - r
    if r == 0: return 1
    if r == 1: return n

    numerator = [n - r + k + 1 for k in range(r)]
    denominator = [k + 1 for k in range(r)]

    for p in range(2,r+1):
        pivot = denominator[p - 1]
        if pivot > 1:
            offset = (n - r) % p
            for k in range(p-1,r,p):
                numerator[k - offset] /= pivot
                denominator[k] /= pivot

    result = 1
    for k in range(r):
        if numerator[k] > 1:
            result *= int(numerator[k])

    return result


x, y = map(int, input().split())
if ((x + y) % 3 != 0):
	print("0")
elif (x > 2*y or y > 2*x):
	print("0")
else:
	n_x = (2 * x - y) // 3
	n_y = (2 * y - x) // 3
	n_xy = (x + y) // 3
	print(cmb(n_xy,n_x)%1000000007)