Phi Euler

#include<bits/stdc++.h>
using namespace std;
const int N = 5e5;
int n, p, phi, tmp;
long long res = 0;
map<long long, int> Map;
long long power(long long a, long long b, long long j)
{
    if (b == 0)
    {
        return 1 % j;
    }
    long long t = power(a, b/2, j);
    if (b % 2 == 0)
    {
        return (t * t) % j;
    }
    else
    {
        return (((t * t) % j) * a) % j;
    }
}
long long inv(int a)
{
    return power(a, phi - 1, p);
}
void read()
{
   cin >> n >> p;
   tmp = p, phi = p;
   for (int i = 2; i * i <= tmp; ++ i)
   {
       if (tmp % i == 0)
       {
           while (tmp % i == 0)
           {
               tmp /= i;
           }
           phi -= phi/i;
       }
   }
   if (tmp != 1)
   {
       phi -= phi/tmp;
   }
}
void solve()
{
   int a;
   for (int i = 1; i <= n; ++ i)
   {
       cin >> a;
       a = (a - 1 + p) % p;
       if (__gcd(a, p) == 1)
       {
           res += Map[inv(a)];
       }
       ++Map[a];
   }
   cout << res;
}
/// inv(x) = x ^ (phi - 1) % p
/// x * inv(x) % p == 1
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    read();
    solve();
}