78D - Archer's Shot - CodeForces Solution

#include <iostream>
#include <cmath>

using namespace std;

long long r, x, y;

bool check(long long x, long long y)
{
    return (((x+1) * (x+1) + 3 * (y+1) * (y+1)) <= (4 * r * r) && ((x+2) * (x+2) + 3 * y * y) <= (4 * r * r));
}


int main()
{
    cin >> r;
    y = 0, x = 0;
    while (check(x, y)) y+=2;
    long long ans = y - 1;
    while (x + 1 <= 2*r)
    {
        if (x != 0)
            ans += 2 * (y + 1);
        x+=3;
        y+=1;
        while (y>=0 && !check(x, y)) y-=2;
    }
    cout <<  ans;
}