#include <stdio.h>
main()
{
double x,y;
scanf("%lf",&x); //读取double要用%lf
if(x<0)
y=0;
else if(x<10)
y=x;
else if(x<20)
y=10;
else if(x<40) //这里少一个if
y=-0.5*x+20;
printf("y=%lf\n",y); //输出double要用%lf
}