#include <bits/stdc++.h>
using namespace std;
#define all(c) (c).begin(),(c).end()
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--)
#define REP(i,m,n) for(int i=(int)(m);i<(int)(n);i++)
#define rep(i,n) REP(i,0,n)
#define iter(c) __typeof((c).begin())
#define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++)
#define pb(a) push_back(a)
#define pr(a) cout<<(a)<<endl
#define PR(a,b) cout<<(a)<<" "<<(b)<<endl
#define F first
#define S second
#define ll long long
bool check(int n,int m,int x,int y){return (x<0||x>=n||y<0||y>=m)?false:true;}
const ll MAX=1000000007,MAXL=1LL<<60,dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
typedef pair<int,int> P;

int main() {
  ll n,m;
  cin >> n >> m;
  ll a=min(n,m),b=max(n,m);
  if(a==1) {
    pr((b-1)/2);
    return 0;
  }
  if(a==b) {
    if(a%2) pr(a/2*(2+a-1)/2);
    else pr(a/2*(1+a-1)/2-1);
  } else {
    if(a==2) pr(b-1);
    else if(a%2) pr(a/2*b+(b+1)/2-1);
    else pr(a/2*b-1);
  }
  return 0;
}