Simple problem

I don’t know what the problem is… So here’s the code

#include <stdlib.h>
#include <gl\glut.h>
#include <gl\GL.h>

void glCross (void)
{
	glClearColor(1, 1, 1, 1);
	glClear(GL_COLOR_BUFFER_BIT);
	glBegin(GL_POLYGON);
		glColor3f(0, 1, 0);
		glVertex2f(0.0, 200.0);
		glVertex2f(400.0, 200.0);
		glVertex2f(200.0, 0.0);
		glVertex2f(200.0, 400.0);
	glEnd();

	glFlush();
}

void main(int argc, char **argv)
{
	glutInit(&argc, argv);
	glutInitWindowPosition(500,500);
	glutInitWindowSize(800,600);
	glutInitDisplayMode(GLUT_RGB | GLUT_SINGLE);
	glutCreateWindow("Cross");
	glutDisplayFunc(glCross);
	glutMainLoop();
}

Solven… i failed with the Vertex positioning.