大佬们SOS

#include<stdio.h>
int main()
{
float cost;
float y;
scanf("%f",&y);
if(0<=y&&y<=50)
cost = y*0.53;
else
{
if(50<=y)
cost = 50*0.53 + (y - 50)*0.58;
}
printf("cost = %.2f\n",cost);
if(y<0)
printf("Invalid Value!");
}
我想在输入负数的时候去掉cost 怎么操作啊