// Croc Champ 2013 - Qualification Round A Spyke Talks
import java.io.*;
import java.io.ObjectInputStream.GetField;
import java.math.*;
import java.text.*;
import java.util.*;
//Codeforces
public class MainCodeforces1 {
private static MyScanner in;
private static PrintStream out;
private static void solve() throws IOException
{
int N = in.nextInt();
long[] id = new long[N];
for (int i = 0; i < N; i++) {
id[i] = in.nextLong();
}
Arrays.sort(id);
int npairs = 0;
int cnt = 1;
boolean mistake = false;
for (int i = 1; i < id.length; i++) {
if (id[i] == 0)
continue;
if (id[i] == id[i - 1])
cnt++;
else
cnt = 1;
if (cnt > 2) {
mistake = true;
break;
}
else if (cnt == 2) {
npairs++;
}
}
if (mistake)
out.print(-1);
else
out.print(npairs);
}
public static void main(String[] args) throws IOException {
// helpers for input/output
boolean LOCAL_TEST = false;// change to false before submitting
out = System.out;
if (LOCAL_TEST) {
in = new MyScanner("E:\\zin.txt");
}
else {
boolean usingFileForIO = false;
if (usingFileForIO) {
// using input.txt and output.txt as I/O
in = new MyScanner("input.txt");
out = new PrintStream("output.txt");
}
else {
in = new MyScanner();
out = System.out;
}
}
solve();
}
// =====================================
static class MyScanner {
Scanner inp = null;
public MyScanner() throws IOException
{
inp = new Scanner(System.in);
}
public MyScanner(String inputFile) throws IOException {
inp = new Scanner(new FileInputStream(inputFile));
}
public int nextInt() throws IOException {
return inp.nextInt();
}
public long nextLong() throws IOException {
return inp.nextLong();
}
public double nextDouble() throws IOException {
return inp.nextDouble();
}
public String nextString() throws IOException {
return inp.next();
}
}
}
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