N, D = map(int, input().split())

V = []
total = 0

for _ in range(N):
    a, b = map(int, input().split())
    V.append((b, a))
    total += a

V.sort(reverse=True)

K = min(D, total)

base = 0
cur = 0
events = {}

for b, a in V:
    take = min(a, K)
    if take == 0:
        break

    base += take * (b // D)

    r = b % D
    if r != 0:
        events[r] = events.get(r, 0) + take
        cur += take

    K -= take

add = cur

for r in sorted(events):
    cnt = events[r]
    cur -= cnt
    add = min(add, r + cur)

print(base + add)