/*
TASK:minjumps
LANG:C++
*/
#include<iostream>
using namespace std;
int main()
{
 int a,b,n,i,g,t;
 cin>>a>>b>>n;
   t=a;
 if(a%2==0&&b%2==0) cout<<"-1"<<endl;
 else
 {if(a+b==n) cout<<"2"<<endl;
  if(a>b) {g=a-b;if(g==n)cout<<"2"<<endl;}
  else {g=b-a;if(g==n)cout<<"2"<<endl;}
  if(b==t+1&&n==b+1&&a+b!=n) cout<<"3"<<endl;
}

     
     
     return 0;
     }
