#include<bits/stdc++.h>
using namespace std;
int main()
{
	double i,c;
	cin>>i;
	if(i<=3)printf("%.2lf",10.00);
	else if(i>3&&i<=5)printf("%.2lf",10.00+(i-3)*1.8);
	else printf("%.2lf",13.6+(i-5)*2.7);
}

C++ :

#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

int main()
{
    double qbj=10,xc,cf;
    cin>>xc;
    if(xc<=3){
        printf("%.2lf",cf);
    }
    if(xc>3&&xc<=5){
        cf=10;
        cf=cf+1.8*(xc-3);
         printf("%.2lf",cf);
    }
    if(xc>5){
        cf=13.6;
        cf=cf+2.7*(xc-5);
         printf("%.2lf",cf);
    }
    return 0;
}