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

    # いい数列かの確認
    S=0
    for i in range(N):
        S+=(1 if i%2==0 else -1)*A[i]

    if S%M:
        return -1

    Mod=998244353
    X=0
    for a in A:
        X=((M-1)*X+a)%Mod

    return (X*pow(M,Mod-2,Mod)+1)%Mod

#==================================================
print(solve())