#include<bits/stdc++.h>
using namespace std;
#define ff       first
#define ss       second
#define MX       1000005
#define mod      1000000007
#define ll       long long
#define pb       push_back
#define pll      pair<ll,ll>
#define endl     "\n"
#define bug(a)   cerr<<#a<<" : "<<a<<endl
#define all(x)   (x).begin(),(x).end()
#define allr(x)  (x).rbegin(),(x).rend()
#define Mul(a,b) (a%mod * b%mod)%mod
#define Add(a,b) (a%mod + b%mod)%mod


int main() {
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);


    int n;
    cin>>n;
    int cnt=0, sum=0;
    for(int i=0; i<n; i++) {
        int x;
        cin>>x;
        sum+=x;
        if(sum%2==0) {
            sum=0;
            cnt++;
        }
    }

    if(sum%2==0) cout<<cnt<<endl;
    else cout<<-1<<endl;

    return 0;
}