D_C = input().split()
Tokyo = True
Total = 0
for i in range(int(D_C[0])):
    money = input().split()
    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])
        else:
            Total += int(money[1])
print(Total)