#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1e9;
int MOD = 1e9+7;
main(){
    ll A,B;
    cin >> A >> B;
    if(A != B){
        cout << (A*B+1)/2 - 1 << endl;
    }else{
        if(A%2){
            cout << (A-1)*(A-1)/4 + A/2 << endl;
        }else{
            cout << A*A/4 - 1 << endl;
        }
    }
}