func_code_string: #include <bits/stdc++.h>   using namespace std;   const int MOD = (int)1e9 + 7;   const int INF = (int)1e9;   const long long LINF = (long long)1e18;   const long double PI = acos((long double)-1);   const long double EPS = 1e-9;   inline long long gcd(long long a, long long b) {     long long r;     while (b) {       r = a % b;       a = b;       b = r;     }     return a;   }   inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }   inline long long fpow(long long n, long long k, int p = MOD) {     long long r = 1;     for (; k; k >>= 1) {       if (k & 1) r = r * n % p;       n = n * n % p;     }     return r;   }   template <class T>   inline int chkmin(T& a, const T& val) {     return val < a ? a = val, 1 : 0;   }   template <class T>   inline int chkmax(T& a, const T& val) {     return a < val ? a = val, 1 : 0;   }   template <class T>   inline T isqrt(T k) {     T r = sqrt(k) + 1;     while (r * r > k) r--;     return r;   }   template <class T>   inline T icbrt(T k) {     T r = cbrt(k) + 1;     while (r * r * r > k) r--;     return r;   }   inline void addmod(int& a, int val, int p = MOD) {     if ((a = (a + val)) >= p) a -= p;   }   inline void submod(int& a, int val, int p = MOD) {     if ((a = (a - val)) < 0) a += p;   }   inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }   inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }   inline int sign(long double x) { return x < -EPS ? -1 : x > +EPS; }   inline int sign(long double x, long double y) { return sign(x - y); }   int p = (int)1e9 + 7;   template <class T, class mul, class dvd>   struct FiniteField {     T x;     template <class G>     T normalize(G x) {       if (x >= 0 && x < p) return x;       x %= p;       if (x < 0)         x += p;       else if (x >= p)         x -= p;       return x;     }     FiniteField() : x(0) {}     template <class G>     FiniteField(const G& y) {       x = normalize(y);     }     FiniteField(const FiniteField& rhs) : x(rhs.x) {}     template <class G>     FiniteField operator()(const G& y) {       x = normalize(y);       return *this;     }     template <class G>     FiniteField operator=(const G& y) {       x = normalize(y);       return *this;     }     int operator==(const FiniteField& rhs) const { return x == rhs.x; }     int operator!=(const FiniteField& rhs) const { return x != rhs.x; }     int operator<(const FiniteField& rhs) const { return x < rhs.x; }     FiniteField operator++(int) {       FiniteField res = *this;       if ((++x) >= p) x -= p;       return res;     }     FiniteField operator++() {       if ((++x) >= p) x -= p;       return *this;     }     FiniteField operator--(int) {       FiniteField res = *this;       if ((--x) < 0) x += p;       return res;     }     FiniteField operator--() {       if ((--x) < 0) x += p;       return *this;     }     template <class G>     FiniteField operator+=(const G& y) {       if ((x += normalize(y)) >= p) x -= p;       return *this;     }     template <class G>     FiniteField operator-=(const G& y) {       if ((x -= normalize(y)) < 0) x += p;       return *this;     }     FiniteField operator+=(const FiniteField& rhs) {       if ((x += rhs.x) >= p) x -= p;       return *this;     }     FiniteField operator-=(const FiniteField& rhs) {       if ((x -= rhs.x) < 0) x += p;       return *this;     }     FiniteField operator+(const FiniteField& rhs) const {       FiniteField res;       res.x = x + rhs.x;       if (res.x >= p) res.x -= p;       return res;     }     FiniteField operator-(const FiniteField& rhs) const {       FiniteField res;       res.x = x - rhs.x;       if (res.x < 0) res.x += p;       return res;     }     template <class G>     FiniteField operator*=(const G& y) {       x = mul()(x, normalize(y), p);       return *this;     }     template <class G>     FiniteField operator/=(const G& y) {       x = dvd()(x, normalize(y), p);       return *this;     }     template <class G>     FiniteField operator*(const G& y) const {       FiniteField res;       res.x = mul()(x, res.normalize(y), p);       return res;     }     template <class G>     FiniteField operator/(const G& y) const {       FiniteField res;       res.x = dvd()(x, res.normalize(y), p);       return res;     }     FiniteField operator*(const FiniteField& rhs) const {       FiniteField res;       res.x = mul()(x, rhs.x, p);       return res;     }     FiniteField operator/(const FiniteField& rhs) const {       FiniteField res;       res.x = dvd()(x, rhs.x, p);       return res;     }     template <class G>     friend FiniteField operator*(const G& y, const FiniteField& rhs) {       FiniteField res;       res.x = res.normalize(y);       return res * rhs;     }     template <class G>     friend FiniteField operator/(const G& y, const FiniteField& rhs) {       FiniteField res;       res.x = res.normalize(y);       return res / rhs;     }     template <class G>     FiniteField operator^=(G k) {       if (k == 0) {         x = 1;         return *this;       }       T t = x;       k--;       while (k > 0) {         if (k & 1) x = mul()(x, t, p);         t = mul()(t, t, p);         k >>= 1;       }       return *this;     }     template <class G>     FiniteField operator^(G k) const {       if (k == 0) return 1;       T r = x, t = x;       k--;       while (k > 0) {         if (k & 1) r = mul()(r, t, p);         t = mul()(t, t, p);         k >>= 1;       }       return r;     }     friend istream& operator>>(istream& is, FiniteField& rhs) {       is >> rhs.x;       return is;     }     friend ostream& operator<<(ostream& os, const FiniteField& rhs) {       os << rhs.x;       return os;     }   };   template <class T>   class mul {    public:     T operator()(const T& x, const T& y, const T& p) {       return (long long)x * y % p;     }   };   template <class T>   class dvd {    public:     T operator()(const T& x, const T& y, const T& p) {       pair<T, T> r = euclid(y, p);       return mul<T>()(x, (r.first % p + p) % p, p);     }       private:     pair<T, T> euclid(T a, T b) {       if (b == 0) return make_pair(1, 0);       pair<T, T> r = euclid(b, a % b);       return make_pair(r.second, r.first - a / b * r.second);     }   };   const int maxn = 100000 + 5;   int m, n;   FiniteField<int, mul<int>, dvd<int> > a[maxn];   map<FiniteField<int, mul<int>, dvd<int> >, int> hs;   void solve() {     cin >> m >> n;     p = m;     for (int i = (0); i < (n); i++) cin >> a[i];     if (n == 1) {       cout << a[0] <<     << 0 <<   n ;       return;     }     if (n == p) {       cout << 0 <<     << 1 <<   n ;       return;     }     FiniteField<int, mul<int>, dvd<int> > sum = 0, sqsum = 0;     for (int i = (0); i < (n); i++) sum += a[i], sqsum += a[i] * a[i], hs[a[i]];     for (int i = (0); i < (n); i++) {       FiniteField<int, mul<int>, dvd<int> > x = a[i];       FiniteField<int, mul<int>, dvd<int> > d = (sum / n - x) * 2 / (n - 1);       if (d == 0) continue;       if (sqsum !=           x * x * n + x * d * n * (n - 1) + d * d * (n - 1) * n * (2 * n - 1) / 6)         continue;       int ok = 1;       for (int j = (0); j < (n); j++) {         if (!hs.count(x + j * d)) {           ok = 0;           break;         }       }       if (ok) {         cout << x <<     << d <<   n ;         return;       }     }     cout << -1 <<   n ;   }   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     solve();     return 0;   }   