Language: C

#pragma region Macros
#include <bits/stdc++.h>
#define SORT(x) sort((x).begin(), (x).end())
#define RSORT(x) sort((x).rbegin(), (x).rend())
#define ALL(x) (x).begin(), (x).end()
#define REV(x) reverse(ALL(x))
#define rep(i, n) for (ll i = 0; i < n; i++)
#define reps(i, m, n) for (ll i = m; i < n; i++)
#define repr(i, m, n) for (ll i = m; i >= n; i--)
#define SP << " " <<
#define INF2 1000000000000000037
#define INF  1000000007
#define lwb(x,n) distance(x.begin(),lower_bound(ALL(x),(n)))
#define upb(x,n) distance(x.begin(),upper_bound(ALL(x),(n)))
#ifdef _MY_DEBUG
#define CHOOSE(a) CHOOSE2 a
#define CHOOSE2(a0,a1,a2,a3,x,...) x
#define debug_1(x1) cout<<#x1<<": "<<x1<<endl
#define debug_2(x1,x2) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<endl
#define debug_3(x1,x2,x3) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<endl
#define debug_4(x1,x2,x3,x4) cout<<#x1<<": "<<x1<<", "#x2<<": "<<x2<<", "#x3<<": "<<x3<<", "#x4<<": "<<x4<<endl
#define de(...) CHOOSE((__VA_ARGS__,debug_4,debug_3,debug_2,debug_1,~))(__VA_ARGS__)
#else
#define de(...)
#endif
using namespace std;
using ll = long long;
using ld = long double;
template<class T = ll> using v = vector<T>;
template<class T = ll> using vv = vector<vector<T>>;
template<class T = ll> using vvv = vector<vector<vector<T>>>;
template<class T> bool maxi(T &a, const T &b){ if(a<b){a=b;return 1;}return 0;}
template<class T> bool mini(T &a, const T &b){ if(b<a){a=b;return 1;}return 0;}
template<class T> ll sumer(const T& a){ return accumulate(ALL(a),0LL); }
template<class T, class U> ostream &operator<<(ostream &os, const pair<T,U> &pe) { os << pe.first << " " << pe.second; return os;}
template<class T> ostream &operator<<(ostream &os, const v<T> &ve) { rep(i,ve.size()) os<< (i?" ":"")<<ve[i]; return os;}
ll topbit(ll a) { return a==0?-1:63-__builtin_clzll(a);}
ll botbit(ll a) { return a==0?64:__builtin_ctzll(a);}
ll popcount(ll a) { return __builtin_popcountll(a);}
#define dame {cout<< retword <<"\n"; return;}
#pragma endregion
//--MODIFS---------------------------------
#pragma region Modifs
#define retword "-1"
#define MOD  1000000007
using P  = pair<ll,ll>;
using TP  = tuple<ll,ll,ll>;
#pragma endregion
//--GLOBAL---------------------------------
template< int mod >
struct ModInt {
    ll x;

    ModInt() : x(0) {}
    ModInt(ll y) : x(y >= 0 ? ( y<mod ? y : y%mod ) : (mod - (-y) % mod) % mod) {}

    ModInt &operator+=(const ModInt &p) {
        if((x += p.x) >= mod) x -= mod;
        return *this;
    }
    ModInt &operator-=(const ModInt &p) {
        if((x += mod - p.x) >= mod) x -= mod;
        return *this;
    }
    ModInt &operator*=(const ModInt &p) {
        x = (int) (1LL * x * p.x % mod);
        return *this;
    }
    ModInt &operator/=(const ModInt &p) {
        *this *= p.inv();
        return *this;
    }

    ModInt operator-() const { return ModInt(-x); }

    ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }
    ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }
    ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }
    ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }
    bool operator==(const ModInt &p) const { return x == p.x; }
    bool operator!=(const ModInt &p) const { return x != p.x; }
    bool operator<(const ModInt &p) const {return x < p.x; }

    ModInt operator+(const ll &q) const {return ModInt(*this) += (ModInt)q; }
    ModInt operator-(const ll &q) const {return ModInt(*this) -= (ModInt)q; }
    ModInt operator*(const ll &q) const {return ModInt(*this) *= (ModInt)q; }
    ModInt operator/(const ll &q) const {return ModInt(*this) /= (ModInt)q; }
    bool operator==(const ll &q) const {return x == q; }
    bool operator!=(const ll &q) const {return x != q; }

    ModInt operator++(){ if(++x == mod) x = 0; return *this; }
    ModInt operator--(){ x = (x == 0 ? mod-1 : x-1); return *this; }
    ModInt operator++(int){ const ModInt res(*this); ++*this; return res; }
    ModInt operator--(int){ const ModInt res(*this); --*this; return res; }

    ModInt inv() const {
        int a = x, b = mod, u = 1, v = 0, t;
        while(b > 0) {
            t = a / b;
            swap(a -= t * b, b);
            swap(u -= t * v, v);
        }
        return ModInt(u);
    }

    ModInt pow(ll n, bool inv=false) const {
        ModInt ret(1), mul(x);
        while(n > 0) {
        if(n & 1) ret *= mul;
            mul *= mul;
            n >>= 1;
        }
        if(inv) ret=ret.inv();
        return ret;
    }

    friend ostream &operator<<(ostream &os, const ModInt &p) {
        return os << p.x;
    }
    friend istream &operator>>(istream &is, ModInt &a) {
        ll t;
        is >> t;
        a = ModInt< mod >(t);
        return (is);
    }
};
using mint = ModInt< MOD >;
mint mpow(ll x, ll n, bool inv=false) {
    mint ret(1), mul(x);
    while(n > 0) {
    if(n & 1) ret *= mul;
        mul *= mul;
        n >>= 1;
    }
    if(inv) ret=ret.inv();
    return ret;
}
vector<mint> powpool;
void makepow(ll num, ll pool, bool inv=false){
    mint now=1;
    if(inv) makepow((now/num).x ,pool);
    rep(i,pool+1){
        powpool.emplace_back(now);
        now*=num;
    }
}
mint poolpow(ll n){
    if(n>=powpool.size()) return powpool[1].pow(n);
    return powpool[n];
}
// .inv() .pow(n,bool) mpow(x,n,bool)  makepow(x,n,bool)->poolpow(n)
//--MAIN-----------------------------------
void Main() {
    ll N,K;
    cin>>N>>K;
    mint ans=0;
    reps(i,K,N+2){
        ans+=(N*2-i+1)*i/2-(i-1)*i/2+1;
    }
    cout<< ans <<"\n";
}
//--START----------------------------------
int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    // ll Qkai; cin>>Qkai; while(Qkai--) Main();
    Main();
}
//-----------------------------------------

Compiler output:
main.c:2:10: fatal error: bits/stdc++.h: No such file or directory
    2 | #include <bits/stdc++.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.