Brasenhas algorithm to draw a circle

/*
Brasenha's Algorithm of drawing circle
*/
#include <GL/gl.h>
#include <GL/glut.h>
#include <bits/stdc++.h>

using namespace std;

double x,y, pk,r;


void display(void){
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0,0.0,0.0);

    pk = 1-r;
    x =0 , y = r;

    while( x <= y ){
        glBegin(GL_POINTS);
        glVertex2i (x,  y);
        glVertex2i (x, -y);
        glVertex2i (-x, y);
        glVertex2i (-x, -y);

        glVertex2i (y,  x);
        glVertex2i (-y, -x);
        glVertex2i (y, -x);
        glVertex2i (-y, x);
        glEnd();
        x++;

        if( pk < 0 ){
            pk = pk + 2 * x  + 1;
        }
        else{
            pk = pk + (2*x - 2*y) + 1;
            y = y-1;
        }
    }


//    glBegin(GL_POLYGON);
//    glVertex2i (40, 60);
//    glVertex2i (60, 60);
//    glVertex2i (60, 40);
//    glVertex2i (40, 40);
//    glEnd();


    glFlush ();
}


void init (void){
glClearColor (1.0, 1.0, 1.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glPointSize(5.0);
glOrtho(-100.0, 100.0, -100.0, 100.0, -100.0, 100.0);

}

int main(int a, char **ar){
glutInit(&a, ar);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (800, 800);
glutInitWindowPosition (100, 100);
glutCreateWindow ("Circle");
init ();
cin >> r;
glutDisplayFunc(display);
glutMainLoop();
return 0;
}