import sys
input = sys.stdin.readline

mod = 998244353
N, M = map(int, input().split())
A = list(map(int, input().split()))
s = sum((-1)**i*x for i, x in enumerate(A)) % M
if s != 0:
    print(-1)
    exit()
ans = 0
for i, x in enumerate(A):
    ans = (ans + x*pow(M-1, N-i-1, mod)) % mod
ans = (ans * pow(M, mod-2, mod) + 1) % mod
print(ans)