# include<iostream> using namespace std; int main(){ float N; scanf("%f",&N); if(0<=N&&N<5){ printf("%.3f",N*-1+2.5); } if(5<=N&&N<10){ printf("%.3f",2-1.5*(N-3)*2); } if(10<=N and N<20){ printf("%.3f",N/2-1.5); } return 0; }