C how to program; 习题3.17 编写一个程序：读入行驶的总公里数和每次加满油后使用的燃料数，计算并显示每次加油后的每加仑平均行驶的里程数，然后还要计算并显示所有燃料的每加仑平均行驶里程数。输入输出样例如图：

#include<stdio.h>
int main(void)
{
    float gallons=0,miles,TotGallons=0,TotMiles=0;
    while (gallons!=-1)
    {
        printf("Enter the gallons used (-1 to end):");
        scanf("%f",&gallons);

        if(gallons!=-1)
        {
            printf("Enter the miles driven:");
            scanf("%f",&miles);
            printf("The miles / gallon for this tank was: %f\n",miles/gallons);
            TotGallons+=gallons;
            TotMiles+=miles;
        }
    }
    printf("\n");
    printf("The overall average miles/gallon was : %f\n",TotMiles/TotGallons);
    return 0;
}