計算階層、C、P、H

#include <iostream>
using namespace std;
unsigned __int64 jei(int n);
int main()
{
    char input;
    int jei_input;
    int a,b;
    cout << "請輸入計算 !、P、C、H，或輸入X離開本程式!!" << endl;
    while (cin >> input)
    {
        switch (input)
        {
        case('C') :
            cout << "請輸入兩個數字a,b，求C(a,b)，(C a取b)" << endl;
            cin >> a >> b;
            cout << "C("<< a << "," << b << ") = " << (jei(a) / (jei(b)*jei(a - b)))<<endl;
            break;
        case('P') :
            cout << "請輸入兩個數字a,b，求P(a,b)，(P a取b)" << endl;
            cin >> a >> b;
            cout << "P(" << a << "," << b << ") = " << jei(a) / jei(a - b) << endl;
            break;
        case('H') :
            cout << "請輸入兩個數字a,b，求H(a,b)，(H a取b)" << endl;
            cin >> a >> b;
            cout << "H(" << a << "," << b << ") = " << jei(a + b - 1) / (jei(a - 1)*jei(b)) << endl;
            break;
        case('!') :
            cout << "輸入一個數字N，求N!" << endl;
            cin >> jei_input;
            cout << jei_input <<"! = "<< jei(jei_input) << endl;
            break;
        case('X') :
            return 0;
        default:
            cout << "輸入錯誤" << endl;
            break;
        }
        cout << "請輸入計算 !、P、C、H，或輸入X離開本程式!!" << endl << endl;
    }
}
unsigned __int64 jei(int n)
    {
        unsigned __int64 sum=1;
        for (int i = 1; i <= n; i++)
        {
            sum *= i;
        }
        return sum;
    }