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- #include<bits\stdc++.h>
- using namespace std;
- int main()
- {
- int t;
- scanf("%d", &t);
- while(t--)
- {
- int n;
- scanf("%d", &n);
- int a[n], i;
- for(i = 0; i < n; i++) scanf("%d", &a[i]);
- int odd = 0, even = 0;
- for(i = 0; i < n; i++)
- {
- if((a[i] % 2) != (i % 2))
- {
- if(a[i] % 2 == 0) even++;
- else odd++;
- }
- }
- if(even == odd) printf("%d\n", even);
- else printf("-1\n");
- }
- }
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