int gcd(int a, int b) {
    int t;
    while(b > 0) {
        t = a;
        a = b;
        b = t % b;
    }

    return a < 0 ? -a : a;
}

size(40cm,5cm);

int ndots = 50;
for(int i = 1; i < ndots; ++i) {
    for(int j = 1; j < ndots; ++j) {
        if (gcd(i, j) == 1) {
            real angle = atan(j/i);
            real height = 1/sqrt(i^2 + j^2);
            real pos = angle - pi/4;

            if(i == j + 2 || j == i + 2) {
                draw((pos,0)--(pos,height), red);
            } else {
                draw((pos,0)--(pos,height));
            }

        }
    }
}