无法获得箭头键的正确值

下面是我的代码:

#include <iostream>
#include <conio.h>
#include <vector>
#include <string>
using namespace std;
int startMenu(vector<string> arr, int pos){
    /* Keyboard
    up    : char(72)
    down  : char(80)
    left  : char(75)
    right : char(77)
    */
    char userChar;
refresh:
    system("cls");
    for (int i = 0; i < arr.size(); i++){
        if (i == pos){
            cout << "> " << arr[i] << endl;
        }
        else{
            cout << "  " << arr[i] << endl;
        }
    }
    userChar = _getch();
    switch (userChar){
    case 0:
    case 0xE0:
        break;
    case 72:
    case 75:
        --pos;
        break;
    case 80:
    case 77:
        ++pos;
        break;
    case 13:
        return pos;
    default:
        break;
    }
    cout << pos << endl;
    if (pos >= arr.size()){
        pos = 0;
    }
    if (pos == -1){
        pos = arr.size() - 1;
        cout << "arr.size() - 1 = " << arr.size() - 1 << endl;
    }
    goto refresh;
}
int main(){
    vector<string> arr;
    arr.push_back("Jamie");
    arr.push_back("Alex");
    startMenu(arr, 0);
}

当pos为0并且我按上/左键时，pos应该被更改为1(因为arr.size() - 1)，然而pos被读取为0(因为箭头键返回"特殊"值)。

我怎么能读-1而不是0后按左/上箭头键时，pos为0?