real x(20),f(20)

	print*,'number of data points'
	read(*,*)m
	print*,'input data points x, f(x)'
	do i=1,m
	read(*,*)x(i),f(i)
	end do
	print*,'input x-coordinate of interpolation point'
	read(*,*)xx
	print*,'largest second derivative in interpolation interval, if known'
	read(*,*)curve

c find closest interval to interpolation point

	do i=1,m-1
 	if(xx.gt.x(i).and.xx.lt.x(i+1)) then
	i1=i
	i2=i+1
	end if
	end do
	print*,'interpolation interval between',x(i1),'and',x(i2)

c find linear interpolation at point xx

	fx=f(i1)+(f(i2)-f(i1))*(xx-x(i1))/(x(i2)-x(i1))
	print*,'interpolated value at point',xx,'=',fx

c estimate error

	if(curve.eq.0.0) curve=max(f(x(i1)),f(x(i2)))
	error=curve*((x(i2)-x(i1))**2.0)/8.0
	print*,'estimated maximum error=',error

	stop
	end