Can't pass to functions with pointer

[mammoth spoon]

I get a run time error saying something about memory locations when I run
this program.How can I make this program work using pointers???

//this program converst miles to km
#include<iostream>
#include "std_lib_facilities.h"

using namespace std;
//==========func declarations====================



void get_mi(double* mi);
void do_math(double* mi, double* km);
void show_math(double* mi, double* km);




//===============main func=========================


int main ()
{ int exit = 0;
do
{
double *mi = 0, *km = 0; //intialize miles and kilometers

get_mi(mi);//get the number of miles

do_math( mi, km);//convert mi to km

show_math(mi, km);//show math to user

cout<<"Enter 1 to repeat, anything else to quit." << endl;
cin >> exit;
}
while(exit == 1 );

return 0;
}


//==============func definitions===========================


void get_mi(double* mi)
{
cout<< "Enter the distance in miles to convert to km: " << endl;
cin >> *mi;
}

void do_math(double* mi, double* km)
{
*km = (*mi) * 1.609; // converts mi to km;
}

void show_math(double* mi, double* km)
{
cout << *mi << "miles is equal to " << *km << "kilometers." << endl;
}

[vlad from moscow]

For example you can write the following way

int main ()
{ int exit = 0;
do
{
double mi = 0, km = 0; //intialize miles and kilometers

get_mi(&mi);//get the number of miles

do_math( &mi, &km);//convert mi to km

show_math(&mi, &km);//show math to user

cout<<"Enter 1 to repeat, anything else to quit." << endl;
cin >> exit;
}
while(exit == 1 );