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#include <iostream> using namespace std; int main() { int n,k; int nul=0; int sum = 0; cin >> n; for (int i =0; i<n*n; i++) { cin >> k; if (k==0) { nul++; } else { sum+=k; } } n = n*n/2; if (nul<=n) { cout << sum; } else { cout << -1; } }