202112-1序列查询(python)

n_N = [int(num) for num in input().split()]
A = [0] + [int(a) for a in input().split()] + [n_N[1]]
S = 0
for i in range(n_N[0] + 1):
    S += i * (A[i+1] - A[i])
print(S)