C :

#include<stdio.h>
int n,m;
int main()
{
	scanf("%d %d",&n,&m);
	printf("%d",n*m/2);
	return 0;
}

C++ :

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

int main(){
	int n,m;
	cin>>n>>m;
	if(n % 2 == 0){
		cout<<n / 2 * m<<endl;
	}else{
		cout<<n / 2 * m + m / 2<<endl;
	}
	
	
	return 0;
}

Python :

sr=input().split()
n=int(sr[0])
m=int(sr[1])
print(n*m//2)