# haru: pypy
import sys
input = sys.stdin.readline
II = lambda : int(input())
MI = lambda : map(int, input().split())
LI = lambda : [int(a) for a in input().split()]
SI = lambda : input().rstrip()
LLI = lambda n : [[int(a) for a in input().split()] for _ in range(n)]
LSI = lambda n : [input().rstrip() for _ in range(n)]
MI_1 = lambda : map(lambda x:int(x)-1, input().split())
LI_1 = lambda : [int(a)-1 for a in input().split()]

mod = 998244353
inf = 1001001001001001001
ordalp = lambda s : ord(s)-65 if s.isupper() else ord(s)-97
ordallalp = lambda s : ord(s)-39 if s.isupper() else ord(s)-97
yes = lambda : print("Yes")
no = lambda : print("No")
yn = lambda flag : print("Yes" if flag else "No")

prinf = lambda ans : print(ans if ans < 1000001001001001001 else -1)
alplow = "abcdefghijklmnopqrstuvwxyz"
alpup = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
alpall = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
URDL = {'U':(-1,0), 'R':(0,1), 'D':(1,0), 'L':(0,-1)}
DIR_4 = [[-1,0],[0,1],[1,0],[0,-1]]
DIR_8 = [[-1,0],[-1,1],[0,1],[1,1],[1,0],[1,-1],[0,-1],[-1,-1]]
DIR_BISHOP = [[-1,1],[1,1],[1,-1],[-1,-1]]
prime60 = [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59]
sys.set_int_max_str_digits(0)
# sys.setrecursionlimit(10**6)
# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')

from collections import defaultdict,deque
from heapq import heappop,heappush
from bisect import bisect_left,bisect_right
DD = defaultdict
BSL = bisect_left
BSR = bisect_right

"""
あんまりきにせずに floor をたせばいい


"""

n, d = MI()
ba = []
for i in range(n):
    a, b = MI()
    ba.append((b, a))
ba.sort(reverse=True)

# d くらいになるまで全体攻撃得


tmp = 0
q = 0
for b, a in ba:
    if q + a >= d:
        # これは全部全体攻撃
        tmp = b
        break
    q += a

eve = []
ans = 0
for b, a in ba:
    if b <= tmp:
        continue
    b -= tmp
    
    g = (b + d - 1) // d
    ans += g * a # とりあえずの回数
    
    ch = b - (g - 1) * d # ここでかわる
    eve.append((ch, a))
    
    
    # print(g, b, d, ch)

tans = ans + tmp

eve.sort()
#
# print(tmp, ans, eve)
pch = 0
for ch, a in eve:
    tmp += ch - pch
    ans -= a
    tans = min(tans, ans + tmp)
    # print(ch, ans, tmp)
    pch = ch

print(tans)