Problem p03145 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vb = vector<bool>;
using vvb = vector<vb>;
using vd = vector<double>;
using vvd = vector<vd>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using pll = pair<ll, ll>;
using tll = tuple<ll, ll>;
using vs = vector<string>;
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, n) range(i, 0, n)
#define range(i, a, n) for (ll i = (a); i < (n); i++)
#define LINF ((ll)1ll << 60)
#define INF ((int)1 << 30)
#define EPS (1e-9)
#define MOD (1000000007ll)
#define fcout(a) cout << setprecision(a) << fixed
#define fs first
#define sc second
#define PI 3.1415926535897932384

int dx[] = {1, 0, -1, 0, 1, -1, -1, 1}, dy[] = {0, 1, 0, -1, 1, 1, -1, -1};
template <class S, class T> ostream &operator<<(ostream &os, pair<S, T> p) {
  os << "[" << p.first << ", " << p.second << "]";
  return os;
};
template <class S> auto &operator<<(ostream &os, vector<S> t) {
  bool a = 1;
  for (auto s : t) {
    os << (a ? "" : " ") << s;
    a = 0;
  }
  return os;
}
template <class S> void tf(bool b, S t, S f) { cout << (b ? t : f) << endl; }
void OUT(bool b) { tf(b, "YES", "NO"); }
void Out(bool b) { tf(b, "Yes", "No"); }
void out(bool b) { tf(b, "yes", "no"); }
template <class T> void puta(T &&t) { cout << t << endl; }
template <class H, class... T> void puta(H &&h, T &&...t) {
  cout << h << ' ';
  puta(t...);
}

int popcnt(int x) { return __builtin_popcount(x); }
int popcnt(ll x) { return __builtin_popcountll(x); }
int bsr(int x) { return 31 - __builtin_clz(x); }
int bsr(ll x) { return 63 - __builtin_clzll(x); }
int bsf(int x) { return __builtin_ctz(x); }
int bsf(ll x) { return __builtin_ctzll(x); }
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

/*他のライブラリを入れる場所*/

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  vi a;
  rep(i, 3) cin >> a[i];
  sort(all(a));
  cout << a[0] * a[1] / 2 << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vb = vector<bool>;
using vvb = vector<vb>;
using vd = vector<double>;
using vvd = vector<vd>;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using pll = pair<ll, ll>;
using tll = tuple<ll, ll>;
using vs = vector<string>;
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, n) range(i, 0, n)
#define range(i, a, n) for (ll i = (a); i < (n); i++)
#define LINF ((ll)1ll << 60)
#define INF ((int)1 << 30)
#define EPS (1e-9)
#define MOD (1000000007ll)
#define fcout(a) cout << setprecision(a) << fixed
#define fs first
#define sc second
#define PI 3.1415926535897932384

int dx[] = {1, 0, -1, 0, 1, -1, -1, 1}, dy[] = {0, 1, 0, -1, 1, 1, -1, -1};
template <class S, class T> ostream &operator<<(ostream &os, pair<S, T> p) {
  os << "[" << p.first << ", " << p.second << "]";
  return os;
};
template <class S> auto &operator<<(ostream &os, vector<S> t) {
  bool a = 1;
  for (auto s : t) {
    os << (a ? "" : " ") << s;
    a = 0;
  }
  return os;
}
template <class S> void tf(bool b, S t, S f) { cout << (b ? t : f) << endl; }
void OUT(bool b) { tf(b, "YES", "NO"); }
void Out(bool b) { tf(b, "Yes", "No"); }
void out(bool b) { tf(b, "yes", "no"); }
template <class T> void puta(T &&t) { cout << t << endl; }
template <class H, class... T> void puta(H &&h, T &&...t) {
  cout << h << ' ';
  puta(t...);
}

int popcnt(int x) { return __builtin_popcount(x); }
int popcnt(ll x) { return __builtin_popcountll(x); }
int bsr(int x) { return 31 - __builtin_clz(x); }
int bsr(ll x) { return 63 - __builtin_clzll(x); }
int bsf(int x) { return __builtin_ctz(x); }
int bsf(ll x) { return __builtin_ctzll(x); }
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

/*他のライブラリを入れる場所*/

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  vi a(3);
  rep(i, 3) cin >> a[i];
  sort(all(a));
  cout << a[0] * a[1] / 2 << endl;
  return 0;
}