import math as m

def ND(x,mean,sd):
z = (x-mean)/(2*m.sqrt(2))
cdf = 0.5*(1 + m.erf(z))
return cdf 

print(f"{ND(19.5,20,2):.4f}")
p1 = ND(20,20,2)
p2 = ND(22,20,2)
print(f"{p2-p1:.4f}")