Language: C++

#include <bits/stdc++.h>
#define rep(i,n) for(ll i=0, i##_len=(n); i<i##_len; ++i)
#define REP(i,num,n) for(ll i=num, i##_len=(n); i<i##_len; ++i)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((ll)(x).size())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define MEMSET(v, h) memset((v), h, sizeof(v))
#define y0 y3487465
#define y1 y8687969
#define j0 j1347829
#define j1 j234892
typedef long long ll;
ll INF = 1e18;
using namespace std;
  

constexpr ll mod = 1e9+7;
void solve();
ll mpow(ll x, ll n){ //x^n(mod) ←普通にpow(x,n)では溢れてしまうため，随時mod計算 2分累乗法だから早い
    ll ans = 1;
    while(n != 0){
        if(n&1) ans = ans*x % mod;
        x = x*x % mod;
        n = n >> 1;
    }
    return ans;
}
class mint {
 private:
  ll _num,_mod;
  mint set(ll num){ 
      _num = num ;
      if(_num>=0) _num%=_mod;
      else _num+=(1-(_num+1)/_mod)*_mod; 
      return *this;
  }
  ll imod(ll n){return mpow(n , _mod-2);}
 public:
  mint(){ _num = 0;_mod=mod; }
  mint(ll num){ _mod = mod; _num = (num+(1LL<<25)*mod) % mod; }
  mint(ll num,ll M){ _mod=M;_num=(num+(1LL<<25)*mod)%_mod; }
  mint(const mint &cp){_num=cp._num;_mod=cp._mod;}
  mint operator= (const ll x){ return set(x); }
  mint operator+ (const ll x){ return mint(_num + (x % _mod) , _mod); }
  mint operator- (const ll x){ return mint(_num - (x % _mod), _mod); }
  mint operator* (const ll x){ return mint(_num * (x % _mod) , _mod); }
  mint operator/ (ll x){ return mint(_num * imod(x) , _mod);}
  mint operator+=(const ll x){ return set(_num + (x % _mod)); }
  mint operator-=(const ll x){ return set(_num - (x % _mod)); }
  mint operator*=(const ll x){ return set(_num * (x % _mod)); }
  mint operator/=(ll x){ return set(_num * imod(x));}
  mint operator+ (const mint &x){ return mint(_num + x._num , _mod); }
  mint operator- (const mint &x){ return mint(_num - x._num , _mod);}
  mint operator* (const mint &x){ return mint(_num * x._num , _mod); }
  mint operator/ (mint x){ return mint(_num * imod(x._num) , _mod);}
  mint operator+=(const mint &x){ return set(_num + x._num); }
  mint operator-=(const mint &x){ return set(_num - x._num); }
  mint operator*=(const mint &x){ return set(_num * x._num); }
  mint operator/=(mint x){ return set(_num * imod(x._num));}
 
  bool operator<(const mint &x){return _num<x._num;}
 
  friend mint operator+(ll x,const mint &m){return mint(m._num + (x % m._mod) , m._mod);}
  friend mint operator-(ll x,const mint &m){return mint( (x % m._mod) - m._num , m._mod);}
  friend mint operator*(ll x,const mint &m){return mint(m._num * (x % m._mod) , m._mod);}
  friend mint operator/(ll x,mint m){return mint(m.imod(m._num) * x , m._mod);}
 
  explicit operator ll() { return _num; }
  explicit operator int() { return (int)_num; }
 
  friend ostream& operator<<(ostream &os, const mint &x){ os << x._num; return os; }
  friend istream& operator>>(istream &is, mint &x){ll val; is>>val; x.set(val); return is;}
};



int main(){
  ll N,K;
  cin >> N >> K;
  
  vector<ll> S1(N);
  vector<ll> S2(N);
  
  vector<ll> A(N);
  
  rep(i,N){
    ll a;
    cin >> A[i];
  }
  
  rep(i,N){
    ll count1 = 0;
    ll count2 = 0;
    rep(j,N){
      if(A[i]>A[j]){
        count1 += 1;
        if(j>i)count2 += 1;
      }
    }
    S1[i] = count1;
    S2[i] = count2;
  }
  
  ll sum1 = 0;
  ll sum2 = 0;
  
  for(ll n: S1){
    sum1 += n;
  }
  
  for(ll n: S2){
    sum2 += n;
  }
  
  // ll ans = ( ( (sum2%MOD) * (K%MOD) )%MOD ) + ( (((sum1%MOD)*(((K-1)%MOD)*(K%MOD))%MOD)%MOD)*modinv(2, MOD)%MOD );
  ll ans = 0;
  
  mint s1,s2,k,km1,n2;
  s1 = sum1;
  s2 = sum2;
  k = K;
  km1 = K-1;
  n2 = 2;

  ans = ( ( sum2 * K )%mod ) + ( ((sum1*((K-1)/2*K)%mod)%mod) );
  ans = (s2 * k + s1*km1*k/n2);
  
  cout << ans << endl;




  return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:127:17: error: cannot convert 'mint' to 'll' {aka 'long long int'} in assignment
  127 |   ans = (s2 * k + s1*km1*k/n2);
      |         ~~~~~~~~^~~~~~~~~~~~~~
      |                 |
      |                 mint
a.cc:69:12: note: explicit conversion function was not considered
   69 |   explicit operator ll() { return _num; }
      |            ^~~~~~~~