余数求和

余数求和

 长记性：1.除化乘逆元 2.每一步都取模 3.注意+mod

#include <bits/stdc++.h>
#include <ctime>
#include <algorithm>
#include <random>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int maxn = 2e4+10;
const int mod = 1e9+7;
const int inf = 0x3f3f3f3f;
#define gok ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define pii pair<int,int>
#define fi first
#define se second
#define pb push_back
#define rep(i,first,second) for(int i=first;i<=second;i++)
#define dep(i,first,second) for(int i=first;i>=second;i--)
#define erep(i,u) for(int i=head[u];~i;i=e[i].nxt)

ll n,ans,inv;
int main()
{
    scanf("%lld",&n);
    ans = (n%mod)*(n%mod)%mod;
    inv=5e8+4;

    for(ll l=1ll,r;l<=n;l=r+1ll){
        r=min(n/(n/l),n);
        ans = (ans+(mod-((((n/l)%mod*(l+r)%mod)%mod*((r-l+1)%mod))%mod*(inv%mod))%mod)%mod)%mod;
    }
    printf("%lld
",ans);
    return 0;
}