Unhandled exception at in ConsoleApplication1.exe: Microsoft C++ exception: std::invalid_argument at memory location

I'm trying to make a basic calculator in Visual studio where the user enters an equation and the equation is solved by taking the equation as a string and then modifying the string to give the solved equation.But when i put the equation i get the error when debugging:

Unhandled exception at 0x00007FF9A1411F28 in ConsoleApplication1.exe: Microsoft C++ exception: std::invalid_argument at memory location 0x000000195B4FF680.

Here's the code:

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

void Calculation_div(string &str);

int main()
{
    string a;
    cin >> a;
    Calculation_div(a);
    cout << a;
}
void Calculation_div(string &str)
{
    std::size_t div_a;
    std::size_t div_r, div_l;
    while(str.find('/')) {
        div_a = str.find('/');
        if (str.find('/', div_a + 1)) {
        div_r = str.find('/', div_a + 1);
        }
        else {
            div_r = str.length();
        }
        if (str.rfind('/', div_a - 1) ) {
            div_l = str.rfind('/', div_a - 1) ;
        }
        else {
            div_l = 0;
        }
        string bi_l = str.substr(div_l, (div_a - div_l));
        string bi_r = str.substr(div_a+1, (div_r - div_a+1));
        int in_l = stoi(bi_l);
        int in_r = stoi(bi_r);
        int res_i = in_l + in_r;
        string res_s = std::to_string(res_i);
        str.replace(div_l, res_s.length(), res_s);
    }
}