friend function

#include <iostream>
using namespace std;
/*
find result in KM between two class using friend function
*/
class dis_m;
class dis_k
{
private:
    float x;
public:
    dis_k()
    {
        x=0;
    }
    void set_k()
    {
        cout<<"enter first dis in KM : ";
        cin>>x;
    }
    friend void cal(dis_k,dis_m);
};
class dis_m
{
private:
    float y;
public:
    dis_m()
    {
        y=0;
    }
    void set_m()
    {
        cout<<"enter second dis in Mile : ";
        cin>>y;
    }
    friend void cal(dis_k,dis_m);
};
void cal(dis_k k,dis_m m)
{
    float sum=0,mile,km,km1;
    mile = m.y;
    km= mile / 0.621;
    km1 = k.x;
    sum = km1+km;
    cout<<"the result in KM : "<<sum<<endl;
}
int main()
{
    dis_k k;
    k.set_k();
    dis_m m;
    m.set_m();
    cal(k,m);
    return 0;
}