Codeforces Round #272 (Div. 2)C. Dreamoon and Sums 数学推公式

C. Dreamoon and Sums

Dreamoon loves summing up something for no reason. One day he obtains two integers a and b occasionally. He wants to calculate the sum of all nice integers. Positive integer x is called nice if  and , where k is some integer number in range[1, a].

By  we denote the quotient of integer division of x and y. By  we denote the remainder of integer division of x andy. You can read more about these operations here: http://goo.gl/AcsXhT.

The answer may be large, so please print its remainder modulo 1 000 000 007 (109 + 7). Can you compute it faster than Dreamoon?

Input

The single line of the input contains two integers a, b (1 ≤ a, b ≤ 107).

Output

Print a single integer representing the answer modulo 1 000 000 007 (109 + 7).

Sample test(s)

input

1 1

output

0

Note

For the first sample, there are no nice integers because  is always zero.

For the second sample, the set of nice integers is {3, 5}.

题意：给你a,b，现在对所有x满足  div(x,b)/mod(x,b) =k  (1<=k<=a)  的x求和 取摸.

题解：  设y=div(x,b),z=mod(x,b)，

可以得到

y=z*k,y*b+z=x,联立得

(kb+1)z=x,

下面用到求和公式，

然后假设k为常量，得到x=b(b-1)*(kb+1)/2，

最后k还原为变量，得到x=b(b-1)/2*[(1+a)a*b/2+a]

///
#include<bits/stdc++.h>
using namespace std ;
typedef long long ll;
#define mem(a) memset(a,0,sizeof(a))
inline ll read()
{
    ll x=,f=;
    char ch=getchar();
    while(ch<''||ch>'')
    {
        if(ch=='-')f=-;
        ch=getchar();
    }
    while(ch>=''&&ch<='')
    {
        x=x*+ch-'';
        ch=getchar();
    }
    return x*f;
}
//****************************************
ll mod =;
#define maxn 100000+5
ll a,b;
int main(){
   a=read(),b=read();
   a=((b*((a*(a+)/)%mod))%mod+a)%mod;b=((b*(b-))/)%mod;
   cout<<(a*b)%mod<<endl;
  return ;
}

代码