#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define RREP(i, n) for (int i = (n); i >= 0; --i)
#define FOR(i, a, n) for (int i = (a); i < (n); ++i)
#define RFOR(i, a, b) for (int i = (a); i >= (b); --i)

#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()

template <class T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    REP(i, SZ(v)) {
        if (i) os << " ";
        os << v[i];
    }
    return os;
}

template <class T, class U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    os << p.first << " " << p.second;
    return os;
}

template <class T>
bool chmax(T& a, const T& b) {
    bool res = a < b;
    if (a < b) a = b;
    return res;
}

template <class T>
bool chmin(T& a, const T& b) {
    bool res = a > b;
    if (a > b) a = b;
    return res;
}

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int, int>;
using PLL = pair<ll, ll>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;

const ll MOD = 1e9 + 7;
const ll MOD998 = 998244353;
const int INF = INT_MAX;
const ll LINF = LLONG_MAX;
const int inf = INT_MIN;
const ll linf = LLONG_MIN;
const ld eps = 1e-9;

template<int m>
struct mint {
    int x;
    mint(ll x = 0) : x(((x % m) + m) % m) {}
    mint operator-() const { return x ? m-x : 0; }
    mint &operator+=(mint r) {
        if ((x += r.x) >= m) x -= m;
        return *this;
    }
    mint &operator-=(mint r) {
        if ((x -= r.x) < 0) x += m;
        return *this;
    }
    mint &operator*=(mint r) {
        x = ((ll)x * r.x) % m;
        return *this;
    }
    mint inv() const { return pow(m-2); }
    mint &operator/=(mint r) { return *this *= r.inv(); }

    friend mint operator+(mint l, mint r) { return l += r; }
    friend mint operator-(mint l, mint r) { return l -= r; }
    friend mint operator*(mint l, mint r) { return l *= r; }
    friend mint operator/(mint l, mint r) { return l /= r; }
    mint pow(ll n) const {
        mint ret = 1, tmp = *this;
        while (n) {
            if (n & 1) ret *= tmp;
            tmp *= tmp, n >>= 1;
        }
        return ret;
    }
    friend bool operator==(mint l, mint r) { return l.x == r.x; }
    friend bool operator!=(mint l, mint r) { return l.x != r.x; }
    friend ostream &operator<<(ostream &os, mint a) {
        return os << a.x;
    }
    friend istream &operator>>(istream &is, mint& a) {
        ll x; is >> x; a = x; return is;
    }
};

using Int = mint<MOD998>;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

    int n, m; cin >> n >> m;
    vi a(n);
    REP(i, n) {
        cin >> a[i];
    }

    ll su = 0;
    RREP(i, n-1) {
        if(i % 2 == 0) {
            su -= a[i];
        } else {
            su += a[i];
        }
    }
    if(su % m != 0) {
        cout << -1 << endl;
        return 0;
    }

    Int mul = 1;
    Int s = 0;
    RREP(i, n-1) {
        s += a[i] * mul;
        mul *= m-1;
    }
    s /= m;
    cout << s + 1 << endl;
}