#importing modules
import math
from __future__ import division
#Variable declaration
e=1.6*10**-19; #charge of electron(coulomb)
r0=2.81*10**-10; #distance between ions(m)
A=1.748; #constant
x=9*10**9; #let x=1/(4*math.pi*epsilon0)
n=9;
#Calculations
U0=-x*A*e**2*(1-(1/n))/(e*r0); #potential energy per ion pair(eV)
#Result
print "potential energy per ion pair is",round(U0/2,3),"eV"
print "answer in the book varies due to rounding off errors"