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: Wrong Answer)

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

using ll = int64_t;
#define int ll

#define FOR(i, a, b) for (int i = int(a); i < int(b); i++)
#define REP(i, b) FOR(i, 0, b)
#define MP make_pair
#define PB push_back
#define EB emplace_back
#define ALL(x) x.begin(), x.end()
auto &errStream = cerr;
#ifdef LOCAL
#define cerr (cerr << "-- line " << __LINE__ << " -- ")
#else
class CerrDummy {
} cerrDummy;
template <class T> CerrDummy &operator<<(CerrDummy &cd, const T &) {
  return cd;
}
using charTDummy = char;
using traitsDummy = char_traits<charTDummy>;
CerrDummy &operator<<(CerrDummy &cd,
                      basic_ostream<charTDummy, traitsDummy> &(
                          basic_ostream<charTDummy, traitsDummy> &)) {
  return cd;
}
#define cerr cerrDummy
#endif
#define REACH cerr << "reached" << endl
#define DMP(x) cerr << #x << ":" << x << endl
#define ZERO(x) memset(x, 0, sizeof(x))
#define ONE(x) memset(x, -1, sizeof(x))

using pi = pair<int, int>;
using vi = vector<int>;
using ld = long double;

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << "(" << p.first << "," << p.second << ")";
  return os;
}

template <class T> ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  REP(i, (int)v.size()) {
    if (i)
      os << ",";
    os << v[i];
  }
  os << "}";
  return os;
}

ll read() {
  ll i;
  scanf("%" SCNd64, &i);
  return i;
}

void printSpace() { printf(" "); }

void printEoln() { printf("¥n"); }

void print(ll x, int suc = 1) {
  printf("%" PRId64, x);
  if (suc == 1)
    printEoln();
  if (suc == 2)
    printSpace();
}

string readString() {
  static char buf[3341000];
  scanf("%s", buf);
  return string(buf);
}

char *readCharArray() {
  static char buf[3341000];
  static int bufUsed = 0;
  char *ret = buf + bufUsed;
  scanf("%s", ret);
  bufUsed += strlen(ret) + 1;
  return ret;
}

template <class T, class U> void chmax(T &a, U b) {
  if (a < b)
    a = b;
}

template <class T, class U> void chmin(T &a, U b) {
  if (b < a)
    a = b;
}

template <class T> T Sq(const T &t) { return t * t; }

#define CAPITAL
void Yes(bool ex = true) {
#ifdef CAPITAL
  cout << "YES" << endl;
#else
  cout << "Yes" << endl;
#endif
  if (ex)
    exit(0);
}
void No(bool ex = true) {
#ifdef CAPITAL
  cout << "NO" << endl;
#else
  cout << "No" << endl;
#endif
  if (ex)
    exit(0);
}

const ll infLL = LLONG_MAX / 3;

#ifdef int
const int inf = infLL;
#else
const int inf = INT_MAX / 2 - 100;
#endif

ll ans;

signed main() {
  ll ab, bc, ca;
  cin >> ab >> bc >> ca;

  ans = (ab * bc) / 2;

  print(ans);

  return 0;
}