#include <bits/stdc++.h>
using namespace std;
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
template <typename T>
inline T gcd(T a, T b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
template <typename T>
inline T lcm(T a, T b) {
  return (a * b) / gcd(a, b);
}
template <typename T>
string toStr(T x) {
  stringstream st;
  st << x;
  string s;
  st >> s;
  return s;
}
template <class T>
void splitStr(const string &s, vector<T> &out) {
  istringstream in(s);
  out.clear();
  copy(istream_iterator<T>(in), istream_iterator<T>(), back_inserter(out));
}
inline int two(int n) { return 1 << n; }
inline int isOnBit(int n, int b) { return (n >> b) & 1; }
inline void onBit(int &n, int b) { n |= two(b); }
inline void offBit(int &n, int b) { n &= ~two(b); }
inline int lastBit(int n) { return n & (-n); }
inline int cntBit(int n) {
  int res = 0;
  while (n && ++res) n -= n & (-n);
  return res;
}
const int dx4[] = {-1, 0, 1, 0};
const int dy4[] = {0, 1, 0, -1};
const int dx8[] = {-1, 0, 1, 0, -1, -1, 1, 1};
const int dy8[] = {0, 1, 0, -1, -1, 1, -1, 1};
int x, n, ok, ans;
map<int, int> M;
int main() {
  ok = 1;
  ans = 0;
  scanf("%d", &n);
  while (n--) {
    scanf("%d", &x);
    if (!x) continue;
    M[x]++;
    if (M[x] == 2) ans++;
    if (M[x] > 2) ok = 0;
  }
  if (ok)
    printf("%d\n", ans);
  else
    printf("-1\n");
  return 0;
}
