import sys
input = sys.stdin.readline

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

B=[0]*(N+5)

for i in range(N-1,-1,-1):
    a,b=AB[i]
    B[i]=max(0,B[i+1]+max(a,b))

ANS=-10**15

NOW=0

for i in range(N):
    a,b=AB[i]

    MAX=max(b,a*(M-1)+b,a*M)

    ANS=max(ANS,NOW+MAX)
    if i>=1:
        ANS=max(ANS,NOW+a+B[i+1])

    NOW+=MAX

print(ANS)