D_C = input().split()
Tokyo = True
Total = 0
Tokyo_money = 0
Kyoto_money = 0-int(D_C[1])
for i in range(int(D_C[0])):
    money = input().split()
    Tokyo_money += int(money[0])
    Kyoto_money += int(money[1])
    if Tokyo:
       if int(money[0]) <= int(money[1])-int(D_C[1]):
            Tokyo = False
            Total += int(money[1])-int(D_C[1])
       else:
           Total += int(money[0])
    else:
        if int(money[1]) <= int(money[0])-int(D_C[1]):
            Tokyo = True
            Total += int(money[0])-int(D_C[1])
        else:
            Total += int(money[1])


    if Kyoto_money > Total:
        Total = Kyoto_money
    elif Tokyo_money > Total:
        Total = Tokyo_money

print(Total)