N, D = map(int, input().split())

dpt = 0

dpk = -D

for i in range(N):
    
    t, k = map(int, input().split())
    
    dpt, dpk = max(dpt + t, dpk + t - D), max(dpt + k - D, dpk + k)
    
print(max(dpt, dpk))