#include<bits/stdc++.h> using namespace std;

int main(){ int n; cin>>n; if(n1){ double l,w; cin>>l>>w; cout<<l*w; }else if(n2){ double a; cin>>a; cout<<aa; }else if(n==3){ double b,h; cin>>b>>h; cout<<bh/2; }else if(n4){ double r,pi=3.14159; cin>>r; cout<<r*pi; }else if(n5){ double c,d,e; cin>>c>>d>>e; cout<<cde/2; } return 0; }