#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

#define reps(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n)   reps(i,0,n)
#define all(x) (x).begin(),(x).end()
#define INF (1000000000)
#define MOD (1000000007)
#define PI (acos(-1))

int main(){
  int a,b;
  cin >> a >> b;
  if((a+b)%2==0){
    cout << (a+b)/2 << endl;
  }else{
    cout << -1 << endl;
  }
}