#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
using mint = modint998244353;
const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, m; cin >> n >> m;
	vector<int>a(n);
	long long sum = 0;
	rep(i, n) {
		cin >> a[i];
		if (0 == i % 2)sum += a[i];
		else sum -= a[i];
	}
	if (0 != sum % m) {
		cout << -1 << endl;
		return -1;
	}
	vector<mint>vx, vy;
	mint x = 1;
	mint y = 0;
	rep(i, n) {
		vx.push_back(x);
		vy.push_back(y);
		mint nx = y * (m - 1);
		mint ny = x + y * (m - 2);
		x = nx;
		y = ny;
	}
	mint ans = 1;
	sum = 0;
	rep(i, n - 1) {
		if (0 == a[i])continue;
		int r = n - sum - a[i];
		int l = n - sum;
		if (0 < r) {
			ans += vx[n - i - 1] + vy[n - i - 1] * (a[i] - 1);
		}
		else {
			ans += vy[n - i - 1] * a[i];
		}
		if (0 == i % 2) sum += a[i];
		else sum -= m - a[i];
		sum %= m;
	}
	cout << ans.val() << endl;
	return 0;
}