def tokyo_and_kyoto(inp=None):
    if inp == None:
        n,d= map(int,input().split())
        inp_l = []
        for _ in range(n):
            inp_l.append(input())
    else:
        inp_l = inp.split("\n")
        n, d = map(int,inp_l.pop(0).split())
    heap = [[[0],0]]
    for _ in range(n):
        l = len(heap)
        a, b = map(int, inp_l.pop(0).split())
        for _ in range(l):
            path, money =heap.pop(0)
            if path[-1] == 0:
                heap.append([path+[0],money+a])
                heap.append([path+[1],money+b-d])
            else:
                heap.append([path+[0],money+a-d])
                heap.append([path+[1],money+b])
    heap.sort(key= lambda x:x[1],reverse=True)
    return heap[0][1]

if __name__ == "__main__":
    print(tokyo_and_kyoto())