#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <iterator>
#include <functional>
#include <complex>
#include <queue>
#include <stack>
#include <cmath>
#include <cassert>
#include <tuple>
using namespace std;
typedef long long LL;
typedef complex<double> CMP;

class VampireTree { public:
	int maxDistance(vector <int> num)
	{
		int d1 = 0, d_else = 0, d_kill = 0;
		for(int d: num)
			if(d == 1)
				d1++;
			else
				d_else++, d_kill+=d-2;
		if(d1-d_kill == 2)
			return d_else + 1;
		else
			return -1;
	}
};

// BEGIN CUT HERE
#include <ctime>
double start_time; string timer()
 { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
 { os << "{ ";
   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
void verify_case(const int& Expected, const int& Received) {
 bool ok = (Expected == Received);
 if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
 cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
#define END	 verify_case(_, VampireTree().maxDistance(num));}
int main(){

CASE(0)
	int num_[] = {1, 2, 1};
	  vector <int> num(num_, num_+sizeof(num_)/sizeof(*num_)); 
	int _ = 2; 
END
CASE(1)
	int num_[] = {2, 2, 2};
	  vector <int> num(num_, num_+sizeof(num_)/sizeof(*num_)); 
	int _ = -1; 
END
CASE(2)
	int num_[] = {1, 1, 1, 1, 4};
	  vector <int> num(num_, num_+sizeof(num_)/sizeof(*num_)); 
	int _ = 2; 
END
CASE(3)
	int num_[] = {1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19};
	  vector <int> num(num_, num_+sizeof(num_)/sizeof(*num_)); 
	int _ = -1; 
END
/*
CASE(4)
	int num_[] = ;
	  vector <int> num(num_, num_+sizeof(num_)/sizeof(*num_)); 
	int _ = ; 
END
CASE(5)
	int num_[] = ;
	  vector <int> num(num_, num_+sizeof(num_)/sizeof(*num_)); 
	int _ = ; 
END
*/

}
// END CUT HERE
