"""

https://yukicoder.me/problems/no/818

2~K-1日目までは同じだろう


1 2 Last
A A A    0<B<A
A A B    0<A<B
B - -    A<0 のものは使用するならこれ?
- - B    存在する？ -> メリット無さそう
- - -

"""

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

now = [0,0]
ans = 0

ed = 1
fi = 0 if M==1 else 1
mid = M - ed - fi

for i in range(N):

    A,B = map(int,input().split())
    n2 = [0,0]

    if 0 <= A <= B:
        n2[0] = max(n2[0], now[0] + fi * A + mid * A + ed * B)
        n2[1] = max(n2[1], max(now) + B)
    elif B <= A and 0 <= A:
        n2[0] = max(n2[0], now[0] + A * M)
        n2[1] = max(n2[1], max(now) + A)
    else:
        n2[0] = max(n2[0], now[0] + max(A*M,B))
        n2[1] = max(n2[1], max(now) + max(A,B))

    # print (n2)
    
    now = n2
    ans = max(ans, max(now))

print (ans)