2986: Non-Squarefree Numbers

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

#define ll long long
const int N=447230;

int p[N+5],u[N+5];
bool vis[N+5];
ll k,ans;

void euler()
{
u[1]=1;
for(int i=2;i<=N;i++){
if(!vis[i]) p[++p[0]]=i,u[i]=-1;
for(int j=1;j<=p[0]&&p[j]*i<=N;j++){
vis[p[j]*i]=1;
if(i%p[j]==0){u[p[j]*i]=0;break;}
u[p[j]*i]=-u[i];
}
}
}

bool check(ll x)
{
ll ret=x;
for(ll i=1;i*i<=x;i++)
ret-=u[i]*(x/(i*i));
return ret>=k;
}

void work()
{
cin>>k;
for(ll l=1,r=3e10;l<=r;){
ll mid=l+r>>1;
if(check(mid)) r=mid-1,ans=mid;
else l=mid+1;
}
printf("%lld\n",ans);
}

int main()
{
euler();
work();
return 0;
}