Getting problem in vi (Ubuntu) in adding two numbers and in output [E488: Trailing characters]. I am using Windows terminal

Question

I am trying to add two numbers in vi using ubuntu terminal on windows. I am getting error E488: Trailing characters. I am new in this usinng a textbook. Can you help me out?

# Script to add two numbers
echo -n "Enter first number"
read num1
echo -n "Enter second number"
read num2
sum= `expr $num1 + $num2`
echo "The additon of $num1 and $num2 is $sum"