n,d=map(int,input().split())
p=[]
q=[]
for i in range(n):
    a,b=map(int,input().split())
    p.append(a)
    q.append(b)

ans=-2**64
for i in range(n):
    for j in range(n):
        if i==j:continue
        pi,qi=p[i],q[i]
        pj,qj=p[j],q[j]
        res=0
        day=d
        res=min(-pi,-pi+qi-pj)
        go=((d-2)//2)*2
        sub=(-pi+qi-pj+qj)*go
        day-=go
        for j in range(1,day+1):
            if j%2==1:
                sub-=pi
                res=min(res,sub)
                sub+=qi
            else:
                sub -= pj
                res = min(res, sub)
                sub += qj
        ans=max(ans,res)
print(ans)