#include <bits/stdc++.h>   using namespace std;   int powmod(int a, int p, int m) {     int64_t ans = 1, cp = a;     for (int i = 0; (1ll << i) <= p; ++i, cp = cp * cp % m)       if ((1ll << i) & p) ans = ans * cp % m;     return ans;   }   int divmod(int64_t a, int b, int m) { return a * powmod(b, m - 2, m) % m; }   const int N = 200000;   int a[N], t[N], b[N], n, m;   unordered_set<int> pres;   vector<int> s;   double et;   pair<int, int> solve() {     {       if (n > 2)         if (rand() & 1)           swap(a[0], a[rand() % (n - 2) + 2]),               swap(a[1], a[rand() % (n - 2) + 2]);         else           swap(a[1], a[rand() % (n - 2) + 2]),               swap(a[0], a[rand() % (n - 2) + 2]);       s.clear();       for (int i = 0; i < n; ++i) s.push_back((a[i] - a[0] + m) % m);       sort(begin(s), end(s));       int dst = 0;       for (int i = 0; i < n; ++i) {         auto t = lower_bound(begin(s), end(s), (a[i] - a[1] + m) % m);         if (t == s.end() || *t != (a[i] - a[1] + m) % m) dst++;       }       int d = divmod((a[1] - a[0] + m) % m, dst, m), heads = 0;       for (int i = 0; i < n; ++i) heads += !pres.count((a[i] + d) % m);       if (!~-heads)         for (int i = 0; i < n; ++i)           if (!pres.count((a[i] + d) % m)) return {a[i], (m - d) % m};     }     return {-1, -1};   }   int main() {     srand(101);     cin.tie(0), ios::sync_with_stdio(0);     int comp = 0;     cin >> m >> n;     for (int i = 0; i < n; ++i) cin >> a[i];     if (n == 1 || n == m) return cout << a[0] <<     << 1, 0;     if (n > m - n) {       sort(a, a + n);       for (int i = 0; i < m; ++i) t[i] = i;       memcpy(a, b, (set_difference(t, t + m, a, a + n, b) - b) * sizeof *b);       comp = 1;       n = m - n;     }     for (int i = 0; i < n; ++i) pres.insert(a[i]);     random_shuffle(a, a + n);     auto ans = solve();     if (!~ans.first)       cout <<  -1 ;     else if (!comp)       cout << ans.first <<     << ans.second <<   n ;     else       cout << (ans.first - ans.second + m) % m <<     << (m - ans.second) % m            <<   n ;     return 0;   }   