N,M=map(int,input().split())
A=list(map(int,input().split()))

score=0

for i in range(N):
    if i%2==0:
        score+=A[i]
    else:
        score-=A[i]

if score%M!=0:
    print(-1)
    exit()

mod=998244353
ANS=0

for i in range(N):
    k=N-1-i
    ANS+=A[k]*pow(M-1,i,mod)*pow(M,mod-2,mod)
    ANS%=mod

print((ANS+1)%mod)