#define _USE_MATH_DEFINES
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <cmath>
#include <map>
#include <iomanip>	
#include <deque>
#include <set>
#include <climits>
#include <memory>
#include <numeric>
#include <utility>


#define rep(i,a,b) for(int i=a;i<b;++i)
#define rrep(i,a,b) for(int i=a;i>=b;--i)
#define fore(i,a) for(auto &i:a)
# define ALL(x) (x).begin(), (x).end()
# define pb push_back
template<class T>bool chmax(T & a, const T & b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T & a, const T & b) { if (b < a) { a = b; return 1; } return 0; }
#define INF INT_MAX/2;
#define PI 3.14159265358979323846264338327950L;
const int INF2 = 1001001001;
typedef long long ll;
using namespace std;
using vi = vector<int>;
using vll = vector<ll>;
ll mod = 1e9 + 7;
//ll llmax = 10000000000000000000ll;
const ll LINF = 1e18;
using namespace std;
using Graph = vector<vector<int>>;



int cnt_digit(ll N)
{
	int digit = 0;

	while (N > 0)
	{
		N /= 10;
		digit++;
	}
	return digit;
}


 // 最大公約数計算
 
ll gcd(ll a, ll b)
{
	if (b == 0) {
		return a;
	}
	else {
		return gcd(b, a % b);
	}
}
// 最小公倍数の計算

ll lcm(ll a, ll b)
{
	ll g = gcd(a, b);

	return a / g * b;
}

struct union_find
{
	vector<int> par, r;
	union_find(int n)
	{
		par.resize(n);
		r.resize(n);
		init(n);
	}


	void init(int n)
	{
		for (int i = 0; i < n; i++) par[i] = i;
		for (int i = 0; i < n; i++) r[i] = 0;
	}

	int find(int x)
	{
		if (par[x] == x)return x;
		else return find(par[x]);
	}

	void unite(int x, int y)
	{
		x = find(x);
		y = find(y);

		if (r[x] < r[y])
		{
			par[x] = y;
		}
		else
		{
			par[y] = x;
			if (r[x] == r[y])
			{
				r[x]++;
			}
		}
	}
};

const int MAX_N = 110000;

int memo[MAX_N];

int rec(int n) {
	if (n == 0)return 0;
	if (memo[n] != -1)return memo[n];

	int res = n;

	for (int pow6 = 1; pow6 <= n; pow6 *= 6)res = min(res, rec(n - pow6) + 1);

	for (int pow9 = 1; pow9 <= n; pow9 *= 9)res = min(res, rec(n - pow9) + 1);

	return memo[n] = res;
}

int calc(int x) {
	int ret = 0;
	while (x % 2 == 0) {
		x /= 2;
		ret++;
	}

	return ret;
}

int a[12][12];
int D;
ll G;
vector<ll> P, C;

//aのp乗を求めるアルゴリズム
long modpow(long a, int p) {
	if (p == 0)return 1;
	if (p % 2 == 0) {
		//pが偶数のとき
		int halfP = p / 2;
		long half = modpow(a, halfP);
		return half * half % mod;
	}
	else {
		//pが奇数の時は、
		//pを偶数にするために1減らす
		return a * modpow(a, p - 1) % mod;
	}

}


//(10*9*8)/(3*2*1);
//10*9*8 -> ansMul
//3*2*1 -> ansDiv;
long calcComb(int a, int b) {
	if (b > a - b) return calcComb(a, a - b);
	long ansMul = 1;
	long ansDiv = 1;
	rep(i, 0, b) {
		ansMul *= (a - i);
		ansDiv *= (i + 1);
		ansMul %= mod;
		ansDiv %= mod;
	}
	//ansMul /ansDivをやりたい
	//ansDivの逆元を使って求めよう。

	long ans = ansMul * modpow(ansDiv, mod - 2) % mod;
	return ans;

}

bool is_war(const vector<int>& x, const vector<int>& y) {
	for (int Z = -100; Z <= 100; Z++) {
		bool is_ok = true;
		rep(i, 0, x.size()) {
			if (x[i] >= Z) {
				is_ok = false;
			}
			rep(i, 0, y.size()) {
				if (y[i] < Z) {
					is_ok = false;
				}
			}
			
		}
		if (is_ok)return false;
		
	}
	return true;
}

//約数列挙
vector<long long> calc_divisor(ll n);

vector<ll> calc_divisor(ll n) {

	vector<ll> res;

	for (ll i = 1ll; i * i <= n; i++) {
		if (n % i == 0) {
			res.push_back(i);

			ll j = n / i;
			if (j != i) {
				res.push_back(j);
			}
		}
	}

	sort(res.begin(), res.end());

	return res;

}

int N;
int ans = 0;

void dfs(ll cur, bool flag3, bool flag5, bool flag7) {

	if (cur > N)return;

	if (flag3 && flag5 && flag7) ans++;

	dfs(cur * 10 + 3, true, flag5, flag7);

	dfs(cur * 10 + 5, flag3, true, flag7);

	dfs(cur * 10 + 7, flag3, flag5, true);

}

int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);
	//入力
	int a,b,c;
	cin >> a >> b >>c;

	cout << a * b / 2 << endl;


	

	

}


Measured memory_bytes: 256