n = int(input())
spyke = list(map(int, input().split()))
d = {}
for x in spyke:
if x in d:
d[x] += 1
elif x != 0:
d[x] = 1
count = 0
flag = True
for x in d.values():
if x == 2:
count += 1
elif x >= 3:
print(-1)
flag = False
break
if flag:
print(count)#include <bits/stdc++.h>
using namespace std;
int main(){
int n,cnt=0,sum=1,m=0;
cin>>n;
int a[n];
for(int i=0; i<n; i++){
cin>>a[i];
if(a[i]==0){
cnt++;
}
}
sort(a,a+n);
int b[n-cnt];
for(int i=cnt; i<n; i++){
b[i-cnt]=a[i];
}
for(int i=1; i<n-cnt; i++){
if(b[i]!=b[i-1]){
sum++;
}
}
int c[sum];
c[0]=1;
for(int i=1; i<sum; i++){
c[i]=0;
}
for(int i=1; i<n-cnt; i++){
if(b[i]!=b[i-1]){
m++;
}
c[m]++;
}
int we=0;
for(int i=0; i<sum; i++){
if(c[i]>2){
cout<<-1;
return 0;
}
else if(c[i]==2){
we++;
}
}
cout<<we;
}145. Binary Tree Postorder Traversal | 94. Binary Tree Inorder Traversal |
101. Symmetric Tree | 77. Combinations |
46. Permutations | 226. Invert Binary Tree |
112. Path Sum | 1556A - A Variety of Operations |
136. Single Number | 169. Majority Element |
119. Pascal's Triangle II | 409. Longest Palindrome |
1574A - Regular Bracket Sequences | 1574B - Combinatorics Homework |
1567A - Domino Disaster | 1593A - Elections |
1607A - Linear Keyboard | EQUALCOIN Equal Coins |
XOREQN Xor Equation | MAKEPAL Weird Palindrome Making |
HILLSEQ Hill Sequence | MAXBRIDGE Maximise the bridges |
WLDRPL Wildcard Replacement | 1221. Split a String in Balanced Strings |
1002. Find Common Characters | 1602A - Two Subsequences |
1555A - PizzaForces | 1607B - Odd Grasshopper |
1084A - The Fair Nut and Elevator | 1440B - Sum of Medians |