newtons forward interpolation

#include<stdio.h>
#include<math.h>
double fact(double x)
{
    if(x==1)
        return 1;
    else
        return (x*fact(x-1));
}
void main()
{
    int n,i,j;
    double x[100],y[100][100],xp,u,l,t,sum=0,a,ans,k;
    printf("enter the number of values to be entered\n");
    scanf("%d",&n);
    printf("enter the values of x and y respectly\n");
    for(i=0;i<n;i++)
        scanf("%lf %lf",&x[i],&y[0][i]);
    printf("the values of x and y are\nx:\t");
    for(i=0;i<n;i++)
        printf("%.0lf   ",x[i]);
    printf("\ny:\t");
    for(i=0;i<n;i++)
        printf("%.4lf   ",y[0][i]);
    printf("\n(dell)y:\t");
    i=0;
    k=n;
    n--;
    do
    {
        i++;
        for(j=0;j<n;j++)
        {
            y[i][j]=y[i-1][j+1]-y[i-1][j];
            printf("%.4lf   ",y[i][j]);
        }
        n--;
        if((i+1)<k)
        printf("\n(dell)%dy:\t",(i+1));
    }while(i<k);
    printf("\nenter the value to be calculated\n");
    scanf("%lf",&xp);
    u=(xp-x[0])/(x[1]-x[0]);
    l=u;
    for(i=1;i<(k-1);i++)
    {
        u=u*(l-i);
        t=fact(i+1);
        a=(u*y[i+1][0])/t;
        sum=sum+a;
    }
    ans=y[0][0]+(l*y[1][0])+sum;
    printf("the answer is %.4lf\n",ans);
}