#include <iostream.h>
#include <fstream.h>
#include <math.h>
void main()

{
	double x[4]={3,1,5,6};
	double f[4]={1,-3,2,4};
	double first[3],second[2],third;
	int i;
	for(i=0;i<3;i++)first[i]=(f[i]-f[i+1])/(x[i]-x[i+1]);// first divided difference
	for(i=0;i<2;i++)second[i]=(first[i]-first[i+1])/(x[i]-x[i+2]);// second divided difference
	third=(second[1]-second[0])/(x[0]-x[3]);// third divided difference
	
	double xx;
	cout<<"Please input interpolation location x : ";
	cin>>xx;
	double ff=f[0]+(xx-x[0])*first[0]
		     +(xx-x[0])*(xx-x[1])*second[0]
		     +(xx-x[0])*(xx-x[1])*(xx-x[2])*third;
	cout<<ff<<endl;
}