How to determine the total resistance of a connection?

Question

C++ / C#

Good day! Please help me, a beginner programmer, complete the program. Problem condition:

A) Three resistances R1, R2, R3 are connected in parallel. Determine the total resistance of the connection. B) Three resistances R1, R2, R3 are connected in series. Determine the total resistance of the connection.

Here are my work:

#include <stdio.h>
#include <conio.h>
#include <iostream>
 
using namespace std;
 
int main()
{
    double Rsum,R1,R2,R3;
    char Sl;
    cout << "Для какого случая необходимо показать решение (А или Б)? ";
    cin>>Sl;
    if (Sl)=='А'
    {
        cout << "Введите R1, R2 и R3: ";
        cin>>R1; cin>>R2; cin>>R3;
            if (R1>0)||(R2>0)||(R3>0)
                {
                Rsum=1/(1/R1+1/R2+1/R3);
                }
                else
                    if (R1<0)||(R2<0)||(R3<0)
                {
                Rsum==0;
                }
    }
                cout << "Сопротивление соединения равно: " << Rsum " Ом. ";
    }
    else
        if (Sl)=='Б'
        {
        cout << "Введите R1, R2 и R3: ";
        cin>>R1; cin>>R2; cin>>R3;
        if (R1>0)||(R2>0)||(R3>0)
                {
                Rsum=R1+R2+R3;
                }
                else
                    if (R1<0)||(R2<0)||(R3<0)
                {
                Rsum==0;
                }
                cout << "Общее соединения равно: " << Rsum " Ом. ";
                getch();
}

Thank you in advance!