n,d = map(int,input().split())
PQ = [list(map(int,input().split())) for i in range(n)]

ans = -10**10
for i in range(n):
    for j in range(i):
        a,b = PQ[i]
        c,d = PQ[j]
        if a+c <= b+d:
            ans = max(ans,min(-a,-a+b-c))
            ans = max(ans,min(-c,-c+d-a))
        else:
            base = (-a-c+b+d)*(d//2-1)
            left = d-2*(d//2-1)
            cand = base
            num = base
            for _ in range(2):
                for k in range(left):
                    if k%2:
                        num -= a
                        cand = min(cand,num)
                        num += b
                    else:
                        num -= c
                        cand = min(cand,num)
                        num += d
                ans = max(ans,cand)
                a,b,c,d = c,d,a,b
print(ans)