How to add an unknown amount of numbers from user input in Linux shell script

I'm trying to add up all numbers inputted by a user, however there is no limit on how many numbers a user can input for the addition. How do I code this in linux shell script?

I have this so far:

firstNumber=0
secondNumber=0
number=0


echo Please enter two numbers to add up

read firstNumber
read secondNumber

echo Would you like to keep adding numbers? YES OR NO
read answer

if answer = YES
then
echo Please add another number
read number
echo $(($firstNumber +$secondNumber + $number))
fi

while answer = NO
do
echo $(($firstNumber + $secondNumber))
done

as @dash-o recommended, a simple entry sequence ended with ENTER is the most simple approach:

#!/usr/bin/env sh

sum=0

echo "Please enter integer numbers to add, or just RETURN to end."

while read -r number && [ -n "$number" ]; do
  if [ "$number" -eq "$number" ] 2>/dev/null; then
    sum=$((sum + number))
    echo "Sum is: $sum"
  else
    echo "$number is not a valid integer. Try again..." >&2
  fi
done

Or to allow multiple integers entry per line:

#!/usr/bin/env sh

# Save the shell's options state
shelloptions="$(set +o)"
# Disable globbing to prevent filename expansion in parameters
set -o noglob

sum=0

echo "Please enter integer numbers to add, or RETURN to end."

# Read lines until empty REPLY
while read -r && [ -n "$REPLY" ]; do

  # Split $REPLY as parameters
  # Globbing is turned-off so filenames will not mess with entries
  # shellcheck disable=SC2086 # Explicitly intended word splitting
  set -- $REPLY

  # Iterate numbers from the parameters array
  for number in "$@"; do

    # If $number is a valid integer
    if [ "$number" -eq "$number" ] 2>/dev/null; then
      sum=$((sum + number))
    else
      echo "$number is not a valid integer." >&2
    fi
  done
  echo "Sum is: $sum"
done

# Restore the shell's options state
eval "$shelloptions"