How do you read input into a variable?

How do you read input into a variable?

I am a beginner with using Python. I am writing a simple program to would
calculate the radius of a circle.
input math
I = input
p = "3.14"
n = "6.28"
I/n
When I run this I get an "integrated function" error because I use
input(). Is there any way I can put the input() data to a variable?
At first I thought that there was a simple syntax error behind all of
this, but after I tried out every single thing that I could, I have given
up all hope on it.