#include 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 ostream& operator<<(ostream& os, const vector& v) { REP(i, SZ(v)) { if (i) os << " "; os << v[i]; } return os; } template ostream& operator<<(ostream& os, const pair& p) { os << p.first << " " << p.second; return os; } template bool chmax(T& a, const T& b) { bool res = a < b; if (a < b) a = b; return res; } template 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; using PLL = pair; using vi = vector; using vll = vector; using vvi = vector; using vvll = vector; 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 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; 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; }