How do a use math operations using a linked list? C++

Question

So I need help with making a linked list use math operations. I haven't got any code to do with the math because I don't know how to convert linked lists to a double. Once a operation is pressed, the linked list store the previous elements and allow the user to input more values then once the '=' is pressed, the program will do the function.

Thank you for the help in advance.

I'll provide my current code and the current output and desired output.

Code:

#include <iostream>
#include <conio.h>
#include <string>
#include <vector>
#include <fstream>
#include <iomanip>
#include <map>
#include <sstream>

using namespace std;

struct Node {
    std::string data{};
    Node* next{ nullptr };
};


void calculator(Node*& head, Node*& tail) {
    bool quit{ false };
    std::string temp{};
    Node* current{ nullptr };
    char c = ' ';




    while (c != 'e')
    {
        char c = ' ';
        c = _getch();

        if (c == '\t' || c == '\n' || c == '\r' || c == ' ') continue;

        if (c == ';') {
            cout << "Returning To Program Menu" << endl;
            return;
        }

        if (c == '+') {
            


        }

        temp = temp + c;

        current = new Node;
        current->data = temp;

        if (tail) {

            tail->next = current;
            tail = current;
        }
        else {
            head = current;
            tail = current;
        }

        std::stringstream ss;
        for (current = head; current; current = current->next) {
            ss << current->data;
        }

        std::cout << "+------------+" << std::endl;
        std::cout << "|" << std::setw(12) << ss.str() << "|" << std::endl;
        std::cout << "+------------+" << std::endl << std::endl;
        temp = "";
    }

}


int main() {
    bool quit = false;
    int choice = 0;
    Node* head = nullptr;
    Node* tail = nullptr;

    calculator(head, tail);

    
    for (auto tmp{ head }; tmp;) {
        tmp = tmp->next;
        delete head;
        head = tmp;
    }
}

Current Output:

+------------+
|           1|
+------------+

+------------+
|          12|
+------------+

+------------+
|         12+|
+------------+

+------------+
|        12+1|
+------------+

+------------+
|       12+12|
+------------+

+------------+
|      12+12=|
+------------+

Desired Output:

+------------+
|           1|
+------------+

+------------+
|          12|
+------------+

+------------+
|           +|
+------------+

+------------+
|           1|
+------------+

+------------+
|          12|
+------------+

+------------+
|           =|
+------------+

+------------+
|          24|
+------------+