Even Array

#include <bits/stdc++.h>

using namespace std;

#define cin(vec) for(auto& i : vec) cin >> i
#define cin_2d(vec, n, m) for(int i = 0; i < n; i++) for(int j = 0; j < m && cin >> vec[i][j]; j++);
#define cout(vec) for(auto& i : vec) cout << i << " "; cout << "\n";
#define cout_2d(vec, n, m) for(int i = 0; i < n; i++, cout << "\n") for(int j = 0; j < m && cout << vec[i][j] << " "; j++);
#define cout_map(mp) for(auto& [f, s] : mp) cout << f << "  " << s << "\n";
#define Time cerr << "Time Taken: " << (float)clock() / CLOCKS_PER_SEC << " Secs" << "\n";
#define fixed(n) fixed << setprecision(n)
//#define ceil(n, m) (((n) / (m)) + ((n) % (m) ? 1 : 0))
#define fill(vec, value) memset(vec, value, sizeof(vec));
#define Num_of_Digits(n) ((int)log10(n)+1)
#define all(vec) vec.begin(),vec.end()
#define rall(vec) vec.rbegin(),vec.rend()
#define sz(x) int(x.size())
#define TC int t; cin >> t;   while(t--)
#define fi first
#define se second
#define Pair pair < int, int >
#define ll long long
#define ull unsigned long long
#define Mod  1'000'000'007
#define OO 2'000'000'000
#define EPS 1e-9
#define PI acos(-1)
#define imin INT_MIN
#define imax INT_MAX
#define getline(s) getline(cin >> ws , s)
#define pb(x) push_back(x)
#define to_decimal(bin) stoi(bin, nullptr, 2)
#define mod_combine(a, b, m) (((a % m) * (b % m)) % m)
#define debug(x) cout << #x << ": " << (x) << "\n";


void Code_Crush(){
    ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
    Time
}

void Solve (){
    int n;
    cin >> n;
    vector < int > nums(n);
    for(auto& i : nums) cin >> i;

    int check = 0;
    for(int i = 0; i < n; i++)
        if(i % 2 == nums[i] % 2) check++;
    if(check == n) return void(cout << 0 << '\n');

    int cnt = 0;
    for(int i = 0; i < n; i++)
        if(i % 2 != nums[i] % 2)
        for(int j = i + 1; j < n; j++)
            if(i % 2 == nums[j] % 2 && j % 2 == nums[i] % 2){
                swap(nums[i], nums[j]), cnt++;
                break;
            }

    if(n == 1 && nums[0] % 2 != 0) cout << -1 << '\n';
    else cout << cnt << '\n';
}


int main (){
    Code_Crush();
    int t = 1;
    //cin >> t;
    while(t--){
        Solve();
    }
    return 0;
}