the reference answer GPT ᴼ ᴾ ᴱ ᴺ ᴬ ᴵ
n, k = map(int, input().split())
d = list(map(int, input().split()))
cost = 0
prev_end = 0
for i in range(n):
if d[i] > prev_end:
days = d[i] - prev_end
subs = (days + k - 1) // k
cost += subs * (k + days)
prev_end += subs * k
print(cost - k)