#include<bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define all(v)  v.begin(),v.end()
const int mod = 1e9 + 7;

void solve(){
    int n;
    cin >> n;
    vector<int> v(n);
    for (int i=0; i<n; i++){
        cin >> v[i];
    }
    int cnt = 0;
    bool flag = 1;
    bool first = 1;
    for (int i=0; i<n; i++){
        if ((v[i] % 2 == 0) and (flag))
            cnt++;
        if ((v[i] % 2 != 0) and (first)){
            flag = 0;
            first = 0;
            continue;
        }
        if ((v[i] % 2 != 0) and (!first)){
            flag = 1;
            first = 1;
            cnt++;
        }
    }
    (flag) ? cout << cnt << endl : cout << -1 << endl;
}

int32_t main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int t = 1;
    //cin >> t;
    while(t--){
        solve();
    }
}