#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    int n,m;
    cin >> n >> m;
    int ans=0;
    long long int c = n*m;
    while(c>1){
        if(n>m){
            ans++;
            c--;
            n--;
        }
        else{
            ans++;
            c--;
            n--;
        }
    }
    cout << ans;
    return 0;
}