求阶梯电价

#include 
int main()
{
	double a, sum;
	scanf("%lf", &a);
    if(a < 0)
	  printf("Invalid Value");
	else if(a <= 50)
	  printf("cost = %.2lf", 0.53 * a);
	else 
	  printf("cost = %.2lf", 0.53 * 50 + 0.58 * (a - 50));
    return 0;	
}