#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define dbg(a,b,c,d) cerr<<a<<"  "<<b<<"  "<<c<<"  "<<d<<endl;
#define kill(a) {cout<<a<<endl;continue;}
#define KILL(a) {cout<<a<<endl;return 0;}
#define debug cerr<<"Error Found"<<endl;
#define mem(a,b) memset(a,b,sizeof(a))
#define lcm(a, b) (a/__gcd(a,b))*b
#define w(t) cin>>t;while(t--)
#define pi  2 * acos(0.0)
#define ll long long int
#define endl "\n"
#define INF (1LL<<61)
int t, cs = 0;
const int mxn = 2e6 + 5, mod = 1e9 + 7;

int32_t main()
{
    //fast;
    int n;
    cin >> n;
    int64_t sum = 0;
    int cnt = 0;
    for(int i = 0; i < n; i++)
    {
        int a;
        cin >> a;
        sum += a;
        if(sum % 2 == 0)cnt++, sum = 0;
    }
    if(sum % 2)cout << -1 << endl;
    else cout << cnt << endl;
}