Replace each '+' character in a string with a '-' character if the '+' is preceded by an odd number?

Question

C++ / C#

The essence of the task in a string (String or StringBuilder) is to replace each '+' character in a string with a '-' character if there is an odd number before the '+'.
I'm trying to do it somehow through the last character (digit), if the number ends with an odd number, then it itself is not even

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace labor4
{
    class Program
    {
        static void Main(string[] args)
        {
            
            string str = Console.ReadLine();
            string[] subs = sb.Split(' ', '+', '-', '*', '/');
            



            foreach (var sub in subs)
            {
                char n = sub.Last();

                

                if (n=='1'|n=='3'|n=='5'|n=='7'|n=='9')
                {
                    
                    
                }

Answer 1

1) Turn the text into a StringBuilder so that characters can be changed
2) Find the plus sign
3) Check what the number is in front of it
4) Change the plus if needed