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;
}1418C - Mortal Kombat Tower | 1382B - Sequential Nim |
1272C - Yet Another Broken Keyboard | 808A - Lucky Year |
1245A - Good ol' Numbers Coloring | 58B - Coins |
1041C - Coffee Break | 507A - Amr and Music |
1041D - Glider | 1486A - Shifting Stacks |
1389B - Array Walk | 71B - Progress Bar |
701A - Cards | 545A - Toy Cars |
1538E - Funny Substrings | 234A - Lefthanders and Righthanders |
1611D - Weights Assignment For Tree Edges | 197A - Plate Game |
1474A - Puzzle From the Future | 6B - President's Office |
1405B - Array Cancellation | 431C - k-Tree |
101A - Homework | 1642C - Great Sequence |
1523B - Lord of the Values | 1406C - Link Cut Centroids |
2409. Count Days Spent Together | 2410. Maximum Matching of Players With Trainers |
1604C - Di-visible Confusion | 997A - Convert to Ones |