Submission #12118636

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Source Code Expand

#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
const ll MOD=1e9+7;
int main()
{
    int n, k;
	cin>>n>>k;
	ll ans=0;
	for(ll i=k; i<=n+1; i++){
		ll s=(ll)n*(n+1)/2-(n-i)*(n-i+1)/2-i*(i-1)/2+1;
		(ans+=s)%=MOD;
	}
	cout<<ans<<endl;
	return 0;
}

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