Language: C

#include <bits/stdc++.h>
using namespace std;
#define int long long
#define All(v) (v).begin(),(v).end()
int dy[8] = {-1,0,1,0,-1,1,1,-1};
int dx[8] = {0,1,0,-1,1,1,-1,-1};
const int mod = 1000000007;
const int inf = mod*mod;
const int d5 = 200100;


template <int mod>
struct ModInt{
    int x;
    ModInt() : x(0) {}
    ModInt(long long y) : x(y >= 0 ? 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.inverse();
        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; }

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

    ModInt pow(int e){
        long long a = 1, p = x;
        while (e > 0){
            if (e % 2 == 0){
                p = (p * p) % mod;
                e /= 2;
            }
            else{
                a = (a * p) % mod;
                e--;
            }
        }
        return ModInt(a);
    }

    friend ostream &operator<<(ostream &os, const ModInt<mod> &p){
        return os << p.x;
    }
    friend istream &operator>>(istream &is, ModInt<mod> &a){
        long long x;
        is >> x;
        a = ModInt<mod>(x);
        return (is);
    }
};

using modint = ModInt<mod>;
modint f[d5];

void makef(){
    f[0] = 1;
    for (int i = 1; i < d5; i++)
        f[i] = f[i - 1] * i;
}

modint nCr(int n, int r){
    if (n < r || r < 0)
        return 0;
    return f[n] / (f[r] * f[n - r]);
}

int beki(int a, int b){
    a %= mod;
    if(b==0)return 1;
    int ans=beki(a,b/2);
    ans=ans*ans%mod;
    if(b%2==1)ans=ans*a%mod;
    return ans;
}

int combination(int a, int b){
    if((a==b)||(b==0)) return 1;
    if(a<b) return 0;
    int ret=1;
    for(int i=0;i<b;i++) {
        ret*=(a-i);
        ret%=mod;
        ret*=beki(i+1,mod-2);
        ret%=mod;
    }
    return ret;
}

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n,k;
    cin>>n>>k;
    modint res=0;
    if(n<k){
        cout<<1<<endl;
        return 0;
    }
    int m=0,M=n;
    for(int i=1;i<k;i++){
        m+=i;
        M+=n-i;
    }
    for(int i=k;i<=n;i++){
        res+=M-m+1;
        m+=i;
        M+=n-i;
    }
    res+=(M-m+1);
    cout<<res<<endl;
}

Compiler output:
main.c:1:10: fatal error: bits/stdc++.h: No such file or directory
    1 | #include <bits/stdc++.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.