import sys
input = sys.stdin.readline

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

A=[list(map(int,input().split())) for i in range(N)]

def calc(x):
    score=x
    for a,b in A:
        score+=(max(0,b-x)+D-1)//D*a

    return score
    

MIN=0
MAX=10**9

while MAX-MIN>200:
    mid1=MIN+(MAX-MIN)//3
    mid2=MIN+(MAX-MIN)//3*2

    m1=calc(mid1)
    m2=calc(mid2)

    if m1>m2:
        MIN=mid1
    else:
        MAX=mid2

ANS=1<<63
for i in range(MIN,MAX+1):
    ANS=min(ANS,calc(i))

print(ANS)