为什么sum加不起来，请教大神！

#include <stdio.h>

int calc_price(float mile, int time)

{

    float sum_price;

    float per_price = 2.3;

    int primary_price = 13;

    if(time >= 5 && time < 23)

    {

        if(mile < 3)

            sum_price = primary_price + mile * per_price + 1;

        else

            sum_price = primary_price + (mile - 3) * per_price + 1;

        printf("您在%d点打车，打车费用为%.2f\n", time, sum_price);

    }

    else

    {

        if(mile < 3)

            sum_price = primary_price + mile * (per_price * 0.2) + 1;

        else

            sum_price = primary_price + (mile - 3) * (per_price * 0.2) + 1;

        printf("您在%d点打车，打车费用为%.2f\n", time, sum_price);

    }

    return sum_price;

}

int main()

{

    printf("打的总费用：%.2f\n",calc_price(12, 9) + calc_price(12, 18));

    return 0;

}