import random

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

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
    cur_v += money[i][cur_place]

print(cur_v)