CCF改名阅读理解大赛吧，渣渣看题目就看了半小时，晕死
最后代码写出来好简单，计算出f(x)帮助理解，最后都注释掉了
n,N = map(int,input().split())
arr = [0]
sum = 0
line = input().split()
for num in line:
arr.append(int(num))
# print(arr)
# farr = [0]*N
for i in range(len(arr)-1):
l = arr[i]
r = arr[i+1]
# print(l,r,i)
# for j in range(l,r):
# farr[j] = i
sum += i * (r - l)
# for j in range(arr[len(arr)-1],N):
# farr[j] = len(arr)-1
# print(farr)
sum += (len(arr)-1) * (N-arr[len(arr)-1])
print(sum)
