#include  <bits/stdc++.h>
using namespace std;
 
#ifdef LOCAL
#include "debug.hpp"
#else
#define bug(...)
#endif

typedef long long ll;
#define endl "\n"
#define ws " "
#define yy cout<<"YES\n";
#define nn cout<<"NO\n";

signed main(){
    int tc=0, tcMax=1;
    // cin >> tcMax;
    while (tc++<tcMax) {
        // bug(tc);
        int n;
        cin>>n;
        int tot = 0;
        vector<int> ara(n);
        for(int i = 0; i < n; i++){
            cin>>ara[i];
            tot+=ara[i];
        }
        if(tot % 2){
            cout<<-1<<endl;
            continue;
        }
        int count = 0;
        int sw = 0;
        for(int i = 0; i < n; i++){
            if(ara[i]%2){
                if(sw == 1){
                    count++;
                    sw = 0;
                }else{
                    sw = 1;
                }

            }else if(ara[i]%2 == 0 && sw == 0){
                count++;
            }
        }
        cout<<count<<endl;
    }
    return 0;
}