About mathematical programming problem...

Code:

#include <stdio.h>int num1;
void a()
{
    printf( "%f\n",3.40*num1 );
}
void b()
{
    printf( "%f\n",11.30*num1 );
}
void c()
{
    printf( "%f\n",4.78*num1 );
}


int main()
{
    while (1)
    {


    int input;


printf ("Enter pairs of item numbers and quantities.\n");
printf ("Enter -1 for the item number to end input.\n");


    scanf ("%d %d", &input ,&num1);
    switch ( input ) {
        case 1:            /* Note the colon, not a semicolon */
            a();
            break;
        case 2:
            b();
            break;
        case 3:
            c();
            break;
        case -1:
            printf( "The total retail value was USD:\n");
            break;
        default:
            printf( "Invalid product code:  %d,input\n" );
            break;


    }}
    getchar();


}

what i write the programming is input (product number)<space>(quantity) then it will print the (price according the product number * quantity) .
but what i want the program is it can total up all the different product price once i type -1.

Code:

Enter pairs of item numbers and quantities.
Enter -1 for the item number to end input.
1 1
2 1
3 1
4 1
Invalid product code:  4
1 1
-1
The total retail value was: USD22.88

This is the output what i need.
how to write the program that once i input "1 1" then it still can input continuously until i input -1 it will show up sum of the price?
Please correct my program, thanks.

Well you need to store
printf( "%f\n",3.40*num1 );
somewhere, not just print it.