超时了，怎么优化

#include <iostream>
using namespace std;
long long ans,a,b,n;
long long sum(long long day)
{
long long X;
long long week=day/7;
long long duoyu=day%7;
if(duoyu>5)
X=week*(5*a+2*b)+5*a + (duoyu-5)*b;
if(duoyu<=5)
X=week*(5*a+2*b) + duoyu*a;
return X;
}
int main()
{
cin >> a>>b>>n;
int l=1,r=1e18;
while(l<=r)
{
int mid=(l+r)>>1;
if(sum(mid)>=n)
{
ans=mid;
r=mid-1;
}
else
{
l=mid+1;
}
}
cout << ans;
return 0;
}