#include<bits/stdc++.h>
using namespace std;
 
int main() {
    int t;
    cin >> t;
 
    while (t--) {
        int n, x = 0, y = 0;
        cin >> n;
 
        for (int i = 0; i < n; ++i) {
            int a;
            cin >> a;
            if (i & 1)
                x += (i ^ a) & 1;
            else
                y += (i ^ a) & 1;
        }
 
        if (x != y)
            cout << -1 << "\n";
        else
            cout << x << "\n";
    }
 
    return 0;
}
 
 

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NAo0CjMgMiA3IDYKMwozIDIgNgoxCjcKNwo0IDkgMiAxIDE4IDMgMAo=

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4
3 2 7 6
3
3 2 6
1
7
7
4 9 2 1 18 3 0