#include <bits/stdc++.h>
using namespace std;

#define fast_io ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define F(i, n) for(int i = 0; i < n; i++)

using ll = long long;
using vi = vector<int>;
using pii = pair<int, int>;

const int LIMIT_INT = 1e5+5, LIMIT_LLONG = 1e9+5;
const long long MOD = 1e9+7;
int tests, n;

void solve()
{
    cin>>n;
    vi rocks(n);
    int high_odd = -1, high_even = -1;
    F(i, n) {
    	cin>>rocks[i];
    	if(i > 0 && i < (n - 1)) {
    		if(rocks[i] % 2 && rocks[i] > 1) high_odd = i;
    		else if(rocks[i] % 2 == 0 && rocks[i] > 1) high_even = i;
    	}
    }
    if(n == 3) {
        if(rocks[1] % 2) cout<<"-1\n";
        else cout<<rocks[1] / 2<<"\n";
    } else if(rocks[n - 2] == 1 && high_odd == -1 && high_even == -1) {
    	cout<<"-1\n";
    }
    else {
        int last_odd = -1;
        int last_even = -1;
        ll op = 0;

    	if(rocks[n - 2] == 1) {
    		if(high_even > -1) rocks[high_even] -= 2;
    		else if(high_odd > -1) rocks[high_odd] -= 2;
    		op++;
    		rocks[n - 2] = 2;
    	}

        for(int i = n - 2; i >= 1; i--) {
            if(rocks[i] % 2) {
                if(last_even == -1 && last_odd == -1) last_odd = i;
                else if(last_odd > -1) {
                    rocks[i]--;
                    rocks[last_odd]--;
                    op += 2;
                    last_even = last_odd;
                    last_odd = -1;
                } else if(last_even > -1)  {
                    rocks[i]++;
                    rocks[last_even] -= 2;
                    op++;
                    last_even = i;
                }
            } else {
                if(last_odd > -1) {
                    rocks[i] -= 2;
                    rocks[last_odd]++;
                    op++;
                    last_even = last_odd;
                    last_odd = -1;
                } else last_even = i;
            }
        }

        for(int i = n - 2; i >= 1; i--) {
            op += (rocks[i] / 2);
        }

        cout<<op<<"\n";
    }
}

int main() {
    fast_io;
    cin>>tests;
    while(tests--) solve();
    return 0;
}