#include<iostream> using namespace std; int main(){ /* int n,m,sum=0; cin>>n; while(n!=0){ sum=sum*10+n%10; n=n/10; } cout<<sum; */ /* int n=0,s=1,m; cin>>m; while(s<m){ n++; s=s*n; } cout<<n; */ double x,y; cin>>x; if(x<5) y=-x+2.5; else if(x<10) y = 2-1.5*(x-3)*2; else if (x<20) y = x/2-1.5; printf("%.3lf",y); return 0; }