#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const ll MOD = 998244353;
const int N = 1000100;
ll fac[N], invfac[N];

ll modpow(ll a, ll b, ll MOD){
	ll ret = 1;
	while (b > 0){
		if(b & 1){
			ret *= a;
			ret %= MOD;
		}
		a *= a;
		a %= MOD;
		b >>= 1;
	}
	return ret;
}

void init(){
	fac[0] = 1;
	for(int i = 1; i < N; i++) fac[i] = fac[i - 1] * i % MOD;
	invfac[N - 1] = modpow(fac[N - 1], MOD - 2, MOD);
	for(int i = N - 2; i >= 0; i--) invfac[i] = invfac[i + 1] * (i + 1) % MOD;
}

ll nCk(int n, int k){
	if(k < 0 || k > n) return 0;
	else return (fac[n] * invfac[k] % MOD) * invfac[n - k] % MOD;
}

void solve(){
	int n, m;
	cin >> n >> m;
	vector<int> A(n);
	for(auto &a :A) cin >> a;
	ll tot = 0;
	for(int i = 0; i < n; i++){
		if(i & 1) tot += A[i];
		else tot -= A[i];
	}
	tot = (tot % m + m) % m;
	if(tot != 0){
		cout << -1 << endl;
		return;
	}

	ll ans = 0;
	ll modm = 0;
	for(int i = 0; i < n - 1; i++){
		ans *= m - 1;
		ans += A[i];
		ans %= MOD;
		modm *= m - 1;
		modm += A[i];
		modm %= m;
	}
	ans -= modm;
	ans *= (m - 1) * modpow(m, MOD - 2, MOD) % MOD;
	ans %= MOD;
	ans += modm + 1;
	ans = (ans % MOD + MOD) % MOD;
	cout << ans << endl;
}

int main(){
	solve();
}