Opengl,C++ : Rotate Squre with respect to the origin

 #include <windows.h>  
 #include <gl/Gl.h>  
 #include <gl/glut.h>  
 #include <cmath>  
 int screenheight = 600;  
 int screenwidth = 800;  
 bool flag = true;  
 int first = 0;  
 double angle = 30;   
 //The angle for the rotation (in degrees)  
 typedef struct{  
   float x;  
   float y;  
 }Point2D;  
 Point2D p1,p2,p3,p4;  
 void DrawPolygonSegment(Point2D pt1, Point2D pt2, Point2D pt3, Point2D pt4){  
     glPointSize(1.0);  
        glBegin(GL_POLYGON);  
             glVertex2i(pt1.x, pt1.y);  
             glVertex2i(pt2.x, pt2.y);  
             glVertex2i(pt3.x, pt3.y);  
             glVertex2i(pt4.x, pt4.y);  
     glEnd();  
     glPointSize(6.0);  
     glBegin(GL_POINTS);  
        glVertex2i(pt1.x, pt1.y);  
     glVertex2i(pt2.x, pt2.y);  
        glVertex2i(pt3.x, pt3.y);  
        glVertex2i(pt4.x, pt4.y);  
     glEnd();  
     glFlush();  
 }  
 Point2D rotate(Point2D p, float ang){  
   ang = ang * 3.14 / 180.0;                 //angle in radians  
   Point2D ptemp;  
   ptemp.x = p.x * cos(ang) - p.y * sin(ang);  
   ptemp.y = p.x * sin(ang) + p.y * cos(ang);  
      return ptemp;  
 }  
 void myMouse()  
 {  
      glClear(GL_COLOR_BUFFER_BIT);  
                               p1.x = 200;  
                               p1.y =200;  
                               p2.x = 400;  
                               p2.y = 200;  
                               p3.x = 400;  
                               p3.y =400;  
                               p4.x=200;  
                               p4.y=400;  
                               DrawPolygonSegment(p1,p2,p3,p4);  
 }  
 void translatepolygon_R()  
 {  
      Point2D Rotated_p1 = rotate(p1,angle);  
   Point2D Rotated_p2 = rotate(p2,angle);  
      Point2D Rotated_p3 = rotate(p3,angle);  
      Point2D Rotated_p4 = rotate(p4,angle);  
   DrawPolygonSegment(Rotated_p1,Rotated_p2,Rotated_p3,Rotated_p4);  
 }  
 void key(unsigned char key, int x, int y)  
 {  
      if(key=='R') glutIdleFunc(translatepolygon_R);  
 }  
 void myDisplay(){  
     glClearColor(1.0f, 1.0f, 1.0f, 0.0f);  
     glClear(GL_COLOR_BUFFER_BIT);  
     glColor3f(0.0f, 0.0f, 0.0f);  
 }  
 int main( int argc, char ** argv ) {  
     glutInit( &argc, argv );  
     glutInitWindowPosition( 0, 0 );  
     glutInitWindowSize( 800, 600 );  
     glutCreateWindow( "My Drawing Screen" );  
        glMatrixMode( GL_PROJECTION );  
     glLoadIdentity();  
     gluOrtho2D( 0, 800, 0, 600 );  
     glViewport(0, 0, 800, 600);  
        glutDisplayFunc( myDisplay );  
     glutDisplayFunc( myMouse );  
        glutKeyboardFunc(key);  
     glutMainLoop();  
     return( 0 );  
 }