这道结论怎么推啊，求条

#include<bits/stdc++.h>
using namespace std;
void print(long long a,long long b){
	if(double(a*b)/2.00==a*b/2)cout<<a*b/2;
	else cout<<a*b/2+1;
}
int main(){
	long long a,b;
	scanf("%lld%lld",&a,&b);
	if(a!=b){
		if(a%2 && b%2){
			print(a,b);
		}
		else if(a%2 || b%2){
			cout<<a*b/2+1;
		}
		else{
			print(a,b);
		}
	}
	else{
		cout<<a*(a-1)/2+a;
	}
	return 0;
}