/*
TASK:minjumps
LANG:C++
*/
# include <iostream.h>
using namespace std;
int main ()
{
    long int a,b,n,br;
    cin >> a >> b >> n;
    br=0;
    if ((a%2==0) && (b%2==0) && (n%2!=0))
    {
    cout << "-1" ;}
    else if (a+b==n)
    {
         cout << "2" ;
    }
    cout << endl;
    return 0;
}
