#include <iostream>

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    
    long long int r, c, ans;
    std::cin >> r >> c;
    if(r%2==1 && r%2==1){
        if(r==c){
            ans = r*c/4;
        }else{
            ans = r*c/2;
        }
    }else{
        if(r==c){
            ans = r/2*c/2-1;
        }else{
            ans = c/2*r-1;
        }
    }
    std::cout << ans << "\n";

    return 0;
}