Chess Board using DDA line drawing algorithm

#include<conio.h> 
#include<graphics.h> 
#include<math.h> 

void ddaline(int x1,int y1,int x2,int y2) 
{ 
    int s,m,dx,dy; 
    float xi,yi,x,y; 

    dx=x2-x1; 
    dy=y2-y1; 

    if(abs(dx)>abs(dy)) 
        s=abs(dx); 
    else 
        s=abs(dy); 

    xi=dx/(float)s; 
    yi=dy/(float)s; 

    x=x1; 
    y=y1; 

    putpixel(x1+0.5,y1+0.5,15); 

    for(m=0;m<s;m++) 
    { 
        x+=xi; 
        y+=yi; 
        putpixel(x+0.5,y+0.5,15); 
    } 
} 


void fill(int x,int y) 
{ 
    int i,j; 
    for(i=x;i<(x+50);i++) 
        ddaline(i,y,i,y+50); 
} 


void main() 
{ 
    int i,j,c=0; 
    int gd=DETECT,gm=DETECT; 

    initgraph(&gd,&gm,""); 
    cleardevice(); 

    ddaline(100,50,100,450); 
    ddaline(100,50,500,50); 
    ddaline(500,50,500,450); 
    ddaline(100,450,500,450); 

    for(i=100;i<500;i+=50) 
    { 
        for(j=50;j<450;j+=50) 
        { 
            if(c%2==0) 
                fill(i,j); 
            c++; 
        } 
        c++; 
    } 
    getch(); 
}