gravity problem..

#include<allegro.h>
#include<winalleg.h>
BITMAP *buffer;
int main()
{
    int v=100;     //vertical speed     
    int y;         //the y variable
    
    allegro_init();
    install_keyboard();
    //set_color_depth(32);
    set_gfx_mode( GFX_AUTODETECT, 800, 600, 0, 0);
    acquire_screen();
    buffer = create_bitmap(800, 600);   
    for(int i=0;i<=800;i++)                         //ground plane basically
    putpixel(buffer,i,500,makecol(200,200,200));
    float time=GetTickCount()/1000;                 //time stuff
    float time1;
    while(!key[KEY_ESC])
    {
                        clear_to_color(buffer, makecol( 0, 0, 0));
                        
                        for(int i=0;i<=800;i++)
                        putpixel(buffer,i,500,makecol(20,20,20));
                        
                        time1=(GetTickCount()/1000)-time;
                        
                        y = 500 - (v*time1) + (5*(time1*time1)); //NEWTONS SECOND LAW y=ut-1/2gt^2
                        
                        putpixel(buffer,400,y,makecol(200,200,200));
                        
                        blit(buffer, screen, 0, 0, 0, 0, 800, 600); 
                        }
                        
                        release_screen();
                        return 0;
                        }
                        END_OF_MAIN();