import random

def does_change(cur_place, cur_day):
    # if (money[cur_day][cur_place] >= money[cur_day][not cur_place]):
    #     print("money[cur_day][cur_place]:",     money[cur_day][cur_place])
    #     print("money[cur_day][not cur_place]:", money[cur_day][not cur_place])
    #     print("!")
    #     return False
    a = 0
    b = - 2 * D
    for i in range(cur_day, N):
        a += money[i][cur_place]
        b += money[i][not cur_place]
        #print("a:", a, "b:", b)
        #print("i:", i)
        if (a < b): return True
    #print("a:", a, "b:", b)
    if (a < b + D): return True
    else: return False

N, D = map(int, input().split())
money = [list(map(int, input().split())) for i in range(N)]

#T = [0 for i in range(N)]
#K = [0 for i in range(N)]
#for i in range(N):
#    T[i], K[i] = map(int, input().split())

# for i in range(N):
#     print(money[i][0], money[i][not 0])

cur_place = 0
cur_v = 0
for i in range(N):
    if (does_change(cur_place, i)):
        cur_place = not cur_place
        cur_v -= D
    # print("cur_place:", int(cur_place))
    cur_v += money[i][cur_place]

print(cur_v)