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 <vector>
#include <iostream>
#include <cmath>
#include <map>
#include <unordered_map>
#include <algorithm>
#include <fstream>
#include <unistd.h>
#include <string>
#include <numeric>
#include <queue>
#include <deque>
#include <sstream>
#include <iomanip>
#include <set>
#include <stack>
#include <cassert>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<string> vs;
typedef vector<vector<string>> vvs;
typedef vector<char> vc;
typedef vector<vector<char>> vvc;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<vvd> vvvd;
typedef pair<int, int> P;
typedef pair<ll, ll> PL;
typedef vector<P> vp;
typedef vector<PL> vpl;
typedef vector<vector<P>> vvp;
typedef vector<vector<PL>> vvpl;
const int INF = 1001001001;
const ll LINF = 1e16;
const double pi = 3.1415926535897932;
const string endstr = "\n";
#define FOR(i, a, b) for(ll i = (a); i < b; i++)
#define REP(i, n) for(ll i = 0; i < n; i++)
#define FORMAP(it, m) for(auto it = m.begin(); it != m.end(); it++)
#define ff first
#define ss second
#define pb push_back

template <typename T>
T gcd(T a, T b) {
    return (a == 0) ? b : gcd(b%a, a);
}
template <typename T>
T lcm(T a, T b) {
    return a / gcd(a, b) * b;
}

bool p_comp_fs(const PL p1, const PL p2){ return p1.first < p2.first;};
bool p_comp_fg(const PL p1, const PL p2){ return p1.first > p2.first;};
bool p_comp_ss(const PL p1, const PL p2){ return p1.second < p2.second;};
bool p_comp_sg(const PL p1, const PL p2){ return p1.second > p2.second;};
template <typename T>
vector<T> uniquen(vector<T> vec){
    vec.erase(unique(vec.begin(), vec.end()), vec.end());
    return vec;
}

//priority_queue<ll, vector<ll>, greater<ll>> gpq;

ll nck(ll n, ll k){
    ll ans = 1;
    FOR(i, 1, k+1){
        ans *= n--;
        ans /= i;
    }
    return ans;
}

int main(){
    ll x, y; cin >> x >> y;
    cout << (x*y) /2 <<endl;
    return 0;
}