“+1”写法与“不+1”写法

//#include <windows.h>//MessageBox(NULL,"下面","上面",1);
#include <iostream>
#include<cmath>
#include <string>
#include <cstring>
#include <algorithm>//sort(a+1,a+n+1);
using namespace std;

//max(n)=1e18-1
//max(r)=3741657384

long long n;
long long sum(long long day){
	long long a,b;
	a=(1+day/7)*(day/7)/2*7;
	b=(day%7)*(day/7+1);
	return a+b;
}

int main() {
	cin>>n;
	long long l=1,r=3741657384,mid;
	while(l<=r){
		mid=(l+r)/2;
		if(sum(mid)<n)l=mid+1;
		else if(sum(mid)>n)r=mid-1;
		else if(sum(mid==n))break;
	}
	cout<<l;
    return 0;
}