c语言，求解答，初学者

#include<stdio.h>

int main(){
    int amount;
    float price, sum = 0;
    scanf("%d,%f", &amount, &price);
    if(price >= 0 && amount >= 0){
        if(amount >= 10)
            sum = amount * price * 0.8;
        else if(amount >= 5)
            sum = amount * price * 0.9;
        else
            sum = amount * price * 0.95;
        printf("%.2f", sum);
    }
    else
        printf("Error!");
    return 0;
}