#include <iostream>
using namespace std;

int main()
{
	long long a,b,n;
	cin>>a>>b>>n;
	const int ans=b;
    int i=1;
	while(n>=0)
	{
	 b=ans*i;
     i++;  
     n-=2;
	 if(b>a-ans)
		{ cout<<b;
		  break; }
	 
	}
	}