C++ Simple Menu Program; difficulty finding middle character of user input

this is only my second time posting here so I hope I am doing this correctly. I need to have the user input a string (usrinput) of any length and make a selection 1 - 4. I have completed 2, 3, and 4, but I cannot figure out how to do selection 1. if the user enters a string "This is a Test" and selects option 1, if odd I need to find the middle letter. if even instead of displaying the middle two letters, I need to let the user know that there is no middle. I feel like I have the function close to being correct, but I'm having a hard time understanding what it means. Any help is greatly appreciated, and I can try to elaborate further if necessary.

#include <iostream>    
#include <iomanip>
#include <string>
using namespace std;

int main()
{
    string usrinput, upper, lower, str;   
    int selection, middle, i;    
    bool menu = true;    
    
    cout << "=============================================================" << endl;
    cout << "Welcome to my program. Enter a sentence and make a selection: " << endl;
    cout << "Enter -999 to exit the program. " << endl;
    cout << "=============================================================" << endl;
    cout << "1. display middle character. " << endl; //if user selects 1, do "..."
    cout << "2. display sentence uppercase " << endl; //if user selects 2, do "..."             
    cout << "3. display sentence lowercase" << endl; //if user selects 3, do "..."
    cout << "4. display sentence backwards" << endl; //if user selects 4, do "..."               

    cout << "Enter a sentence: " << endl;        
    getline(cin, usrinput); //sentence input
    
    
    while (menu == true)
    {
    cout << "Make a selection: " << endl; // if selection is 1 - 4 || -999 (good input) anything <1 or >5 (bad input, loop until selection = 1 - 4
    cin >> selection;

    //Step 1. Input Validation
    while (selection != 1 && selection != 2 && selection != 3 && selection != 4 && selection != -999) //If the selection is not 1 - 4 || -999 loop until selection is valid. 
    {
        cout << "Invalid Entry. Please make another selection: " << endl; 
        cin >> selection;
    }

    if (selection == 1) // if the user enters 1: show middle character if there is one / let the user know there isn't one.
    {
        cout << "Middle: " << endl;
        cout << "=======" << endl;   


        if (((i = usrinput.length() /2 % 2) == 1))
        {
            cout << usrinput[i];
            cout << endl;
        }

        else if (((i = usrinput.length() / 2) % 2) >= 1)

        {
            cout << "There is no middle";
            cout << endl;
        }           

    }