#include <iostream>
#include <algorithm>
using namespace std;

const long long md = 998244353;

int c[200009];
long long d[200009][2];

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n, m; cin >> n >> m;
	d[n][0] = 1;
	for (int i = n - 1; i >= 1; i--) {
		c[i] = ((n - i) & 1);
		d[i][0] = ((d[i + 1][1]) * (m - 1)) % md;
		d[i][1] = (d[i + 1][1] * (m - 2) + d[i + 1][0]) % md;
	}
	int s = 0;
	long long ans = 1;
	for (int i = 1; i <= n; i++) {
		int a; cin >> a;
		if (i & 1) s = (s + a) % m;
		else s = (s - a + m) % m;
		int ds = (s - c[i] + m) % m;
		if (0 < ds && ds <= a) ans = (ans + d[i][0] + d[i][1] * (a - 1)) % md;
		else ans = (ans + d[i][1] * a) % md;
	}
	if (s) cout << -1 << '\n';
	else cout << ans << '\n';
	return 0;
}