n,d=map(int,input().split())
food=[]
for _ in range(n):
    p,q=map(int,input().split())
    food.append([-p,q,q-p])
food.sort()
las=food[-1][0]
ans=-10**9+1
for i in range(n):
    for j in range(n):
        if i==j:
            continue
        T=food[i][2]+food[j][2]
        tmp=min(food[i][0],food[i][2]+food[j][0])
        if T<0:
            if tmp==food[i][0]:
                if d%2==0:
                    tmp=T*(d//2-1)+food[i][0]
                else:
                    tmp=T*((d-1)//2)+las
            else:
                if d%2==0:
                    tmp=T*(d//2-1)+tmp
                else:
                    tmp=T*((d-1)//2)+min(las,-food[j][1])
        ans=max(ans,tmp)
print(ans)