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

typedef long long ll;
typedef unsigned long long ull;
typedef double dd;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;

const int mx=1e6+11;
const double PI = acos(-1);
#define MOD 1000000007

#define nl '\n'
#define pb push_back
#define F first
#define S second
#define I insert
#define mp make_pair

#define sz(x) x.size()
#define tt int t; cin>>t; while(t--)
#define be(a) (a).begin(),(a).end()
#define rbe(a) (a).rbegin(),(a).rend()
#define mem(a,b) memset(a,b,sizeof(a))
#define yes cout<<"YES"<<nl
#define no cout<<"NO"<<nl

#define su ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define file freopen("input.txt","r",stdin);freopen("output.txt","w",stdout);
#define frac cout.unsetf(ios::floatfield); cout.precision(10); cout.setf(ios::fixed,ios::floatfield);

template < typename F, typename S >
ostream& operator << ( ostream& os, const pair< F, S > & p ) {
            return os << "(" << p.first << ", " << p.second << ")";
}

template < typename T >
ostream &operator << ( ostream & os, const vector< T > &v ) {
            os << "{";
                for(auto it = v.begin(); it != v.end(); ++it) {
                                if( it != v.begin() ) os << ", ";
                                        os << *it;
                                            }
                    return os << "}";
}

template < typename T >
ostream &operator << ( ostream & os, const set< T > &v ) {
            os << "[";
                for(auto it = v.begin(); it != v.end(); ++it) {
                                if( it != v.begin() ) os << ", ";
                                        os << *it;
                                            }
                    return os << "]";
}

template < typename T >
ostream &operator << ( ostream & os, const multiset< T > &v ) {
            os << "[";
                for(auto it = v.begin(); it != v.end(); ++it) {
                                if( it != v.begin() ) os << ", ";
                                        os << *it;
                                            }
                    return os << "]";
}

template < typename F, typename S >
ostream &operator << ( ostream & os, const map< F, S > &v ) {
            os << "[";
                for(auto it = v.begin(); it != v.end(); ++it) {
                                if( it != v.begin() ) os << ", ";
                                        os << it -> first << " = " << it -> second ;
                                            }
                    return os << "]";
}

#define dbg(args...) do {cerr << #args << " : "; faltu(args); } while(0)

void faltu () {
            cerr << endl;
}

template <typename T>
void faltu( T a[], int n ) {
            for(int i = 0; i < n; ++i) cerr << a[i] << ' ';
                cerr << endl;
}

template <typename T, typename ... hello>
void faltu( T arg, const hello &... rest) {
            cerr << arg << ' ';
                faltu(rest...);
}


int gcd(int a,int b){return b ? gcd(b,a%b) : a;}
int lcm(int a,int b){return (a*( b/ gcd(a,b)));}

void f()
{
  int a[4];
  for(int i=0;i<3;i++)
  {
    cin>>a[i];
  }
  sort(a,a+3);
  cout<<a[0]*a[1]/2<<nl;
}

int main()
{
    su;
    f();
    return 0;
}


Measured memory_bytes: 3552