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

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