import sys
def f(a, d):
a -= 1
return a // d + (1 if a % d else 0)
n, d = map(int, sys.stdin.readline().split())
mx = ans = 0
for _ in range(n):
a, b = map(int, sys.stdin.readline().split())
x = f(a, d) * f(b, d)
if mx < x:
mx = x
ans = a * b
elif mx == x and ans < a * b:
ans = a * b
print(ans)