Калькулятор Си, есть наработки

#include<iostream>
#include<math.h>
 
int factorial(int a)
{
    if (a == 0) return 1;
    else return a * factorial(a - 1);
}
 
void main(void)
{
    float k = 0,a = 0,c = 0;
    int b = 0;
fg: ;// метка
    std::cout << "vvedin 1 chislo: ";
    std::cin >> a;
    std::cout << "\n1 +";
    std::cout << "\n2 -";
    std::cout << "\n3 *";
    std::cout << "\n4 /";
    std::cout << "\n5 ostacha ot deleniya";
    std::cout << "\n6 okryglit do bolshego";
    std::cout << "\n7 okryglit do menshogo";
    std::cout << "\n8 korin";
    std::cout << "\n9 lg(x)";
    std::cout << "\n10 sin(x)";
    std::cout << "\n11 cos(x)" ;
    std::cout << "\n12 x^(y)" ;
    std::cout << "\n13 x^2";
    std::cout << "\n14 x^3";
    std::cout << "\n15 x!";
    std::cout << "\n16 1/x";
    std::cout << "\n17 exit" << std::endl;
 
    std::cin >> b;
    if(b == 6 && (a - int(a)) > 0) k = int(a) + 1;
    else if(b == 6) { if ((a - int(a)) == 0) k = a;}
    else if(b == 7) {if ((a - int(a)) >= 0) k = int(a);}
    else if(b == 8) {if( a > 0) k=sqrt(a);}
    else if(b == 9) {if( a > 0) k=log10(a);}
    else if(b == 10) k = sin(a);
    else if(b == 11) k = cos(a);
    else if(b == 13) k = pow(a, 2);
    else if(b == 14) k = pow(a, 3);
    else if(b == 15) {if (a >= 0) k = factorial(a);}
    else if(b == 16) {if (a != 0) k = 1 / a;}
    else if(b == 17) return;
    else 
    {
        std::cout << "vvedin 2 chislo: ";
        std::cin >> c;
        if (b == 1) k=a+c;
        else if(b == 2) k = a - c;
        else if(b == 3) k = a * c;
        else if(b == 4) k = a / c;
        else if(b == 5) k = ((a / c) - int(a / c))*c;
        else if(b == 12) k = pow(a, c);
    }
    std::cout << "otvet = " << k << std::endl << std::endl;
    goto fg;
}