Language: C

// Kevin Mathew T
// Birla Institute of Technology, Mesra
// LinkedIn - https://www.linkedin.com/in/KevinMathewT/
// GitHub - https://github.com/KevinMathewT
// CodeForces - https://codeforces.com/profile/KevinMathew
// CodeChef - https://www.codechef.com/users/KevinMathew
// HackerRank - https://www.hackerrank.com/KevinMathew

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double ld;
#define F first
#define S second
#define fi(a, b) for(ll i=(a);i<=b;i++)
#define fj(a, b) for(ll j=(a);j<=b;j++)
#define fid(a, b) for(ll i=(a);i>=b;i--)
#define fjd(a, b) for(ll j=(a);j>=b;j--)

// #include <ext/pb_ds/assoc_container.hpp> // Common file 
// #include <ext/pb_ds/tree_policy.hpp> 
// #include <functional> // for less 
// #include <iostream> 
// using namespace __gnu_pbds; 
// using namespace std; 
// typedef tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
// find_by_order(k) : Value at kth index in sorted array. 
// order_of_key(k) : Index of the value k.

template<class T> ostream& operator<<(ostream &os, vector<T> V) {
    os << "[ ";
    for(auto v : V) os << v << " ";
    os << "]";
	return os;
}
template<class T> ostream& operator<<(ostream &os, set<T> S){
    os << "{ ";
    for(auto s:S) os<<s<<" ";
    return os<<"}";
}
template<class T> ostream& operator<<(ostream &os, multiset<T> S){
    os << "{ ";
    for(auto s:S) os<<s<<" ";
    return os<<"}";
}
template<class L, class R> ostream& operator<<(ostream &os, pair<L,R> P) {
    return os << "(" << P.first << "," << P.second << ")";
}
template<class L, class R> ostream& operator<<(ostream &os, map<L,R> M) {
    os << "{ ";
    for(auto m:M) os<<"("<<m.F<<":"<<m.S<<") ";
    return os<<"}";
}
template<class L, class R> ostream& operator<<(ostream &os, unordered_map<L,R> M) {
    os << "{ ";
    for(auto m:M) os<<"("<<m.F<<":"<<m.S<<") ";
    return os<<"}";
}


const ll N = 1, M = 1e9 + 7;
ll n, k;

ll power(ll a, ll x) {
	a = a % M;
	if(x == 0) return 1;
	if(x == 1) return a;
	if(x % 2 == 0) return power((a * a) % M, x / 2);
	return (a * (power((a * a) % M, x / 2))) % M;
}

void solve(){
	cin >> n >> k;
	ll tot = 0;
	fi(k, n) {
		ll z = (n - i + M) % M;
		// ll t1 = (((n * (n + 1)) % M) * power(2, M - 2)) % M;
		// ll t2 = ((z * (z + 1)) % M * power(2, M - 2)) % M;
		// ll t3 = ((i * (i + 1)) % M * power(2, M - 2)) % M;
		// cout << (((n * (n + 1)) / 2) - ((z * (z + 1)) / 2)) << " ";
		// cout << ((i * (i - 1)) / 2) << "\n";
		tot = (tot +((((((((n * (n + 1)) % M)) * power(2, M - 2)) % M)) - ((((((z * (z + 1)) % M)) * power(2, M - 2)) % M))) - ((((((((i * (i - 1)) % M)))) * power(2, M - 2)) % M)) + 1 + M) % M) % M;
		// tot = (tot + (((t1 - t2 + M) % M) - t2 + M) % M) % M;
		// cout << tot << "\n";
	}

	cout << tot << "\n";
}

int main(){
	//freopen("input.txt", "r", stdin);		//Comment
	//freopen("output.txt", "w", stdout);		//this out.
	ios::sync_with_stdio(false);			//Not
	cin.tie(NULL);							//this.
	cout.tie(0);							//or this.

	// ll T;
	// cin >> T;
	// while(T--)
		solve();

	return 0;
}

Compiler output:
main.c:9:10: fatal error: bits/stdc++.h: No such file or directory
    9 | #include <bits/stdc++.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.