Problem:
Score : 100 points

Problem Statement

There is a right triangle ABC with ∠ABC=90°.

Given the lengths of the three sides, |AB|,|BC| and |CA|, find the area of the right triangle ABC.

It is guaranteed that the area of the triangle ABC is an integer.

Constraints

1 \leq |AB|,|BC|,|CA| \leq 100

All values in input are integers.

The area of the triangle ABC is an integer.

Input

Input is given from Standard Input in the following format:

|AB| |BC| |CA|

Output

Print the area of the triangle ABC.

Sample Input 1

3 4 5

Sample Output 1

6

This triangle has an area of 6.

Sample Input 2

5 12 13

Sample Output 2

30

This triangle has an area of 30.

Sample Input 3

45 28 53

Sample Output 3

630

This triangle has an area of 630.

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define FFOR(i, a, b) FOR(i, a, b+1)
#define REP(i, n) FOR(i, 0, n)
#define RREP(i, n) FOR(i, 1, n+1)
#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define LEN(x) (int)(x).size()
#define DUMP(x) cerr<<__LINE__<<' '<<#x<<"="<<(x)<<endl;
#define pb push_back
#define eb emplace_back
#define mp make_pair

using namespace std;
using lint = long long;
using pii =  pair<int, int>;
using pll =  pair<lint, lint>;
template <typename T> using vc = vector<T>;
template <typename T> using vvc = vector<vector<T>>;
template <typename T> inline bool chmax(T &a, T b){if(a<b){a = b; return true;} return false;}
template <typename T> inline bool chmin(T &a, T b){if(a>b){a = b; return true;} return false;}

constexpr lint ten(int n) {return n==0 ? 1 : ten(n-1)*10;}

class Task{
public:
    void solve(istream& in, ostream& out){
        int ab, bc, ca;
        in>>ab>>bc>>ca;
        out<<ab*bc/2<<'\n';
    }
};

int main(){
    istream& in(cin);
    ostream& out(cout);
    ios_base::sync_with_stdio(false);
    in.tie(nullptr), out.tie(nullptr);
    Task solver;
    solver.solve(in, out);
    return 0;
}