#include<stdio.h>
int main()
{
    int a,b,c,d;
    scanf("%d %d %d",&a,&b,&c);
    d=a*(b+c-a);
    printf("%d",d);
    return 0;

}

#include <stdio.h>

int main() {

int a;
int b;
int x;

scanf("%d %d", &a, &b);

x = a*b/2;

printf("%d", x);

return 0;

}

#include<stdio.h>
int main()
{
    int a,b,c;
    scanf("%d %d",&a,&b);
    c=a*b/2;
    printf("%d",c);
    return 0;

}

#include <iostream> using namespace std; int main() { int a,b; cin>>a>>b; cout<<a*b/2<<endl; return 0;

}