How to make a program display triangle, not a triangle or right triangle?

I'm doing a program for my computer programming class and it's about triangles. We have to input 3 measurements for each angle. If it doesn't add up to 180 then get it to print out "This is not a triangle.". If it adds up to 180 then get it to print out "This is a triangle.". And if it adds up to 180 and one of the angles is 90 then get it to print out "This is a right triangle.". We can only have one statement i.e. it can't say it's a triangle and right triangle.

This is what I have and I know it's wrong. If you could explain what's wrong it would help.

50 input "First angle measurement";a1

60 input "second angle measurement";a2

70 input "Third angle measurement";a3

80 let sum=a1+a2+a3

85 cls

90 if sum<>180 then print "This is not a triangle."

100 if sum=180 then print "This is a triangle."

105 if sum=180 and a1=90 then print "This is a right triangle."

110 if sum=180 and a2=90 then print "This is a right triangle."

115 if sum=180 and a3=90 then print "This is a right triangle."