#include <algorithm>
#include <iostream>

using namespace std;

int main(int argc, const char* argv[])
{
	int N;
	cin >> N;
	int A[N], B[N];
	for (int i = 0; i < N; i++) cin >> A[i] >> B[i];
	int W[N];
	for (int i = 0; i < N; i++) W[i] = A[i] + 4 * B[i];
	sort(W, &W[N]);
	int w	  = W[N - 1];
	size_t cnt = 0;
	for (int i = 0; i < N - 1; i++) {
		int gap = w - W[i];
		if (gap % 2 == 1) {
			cout << -1 << endl;
			return 0;
		}
		cnt += gap;
	}
	cnt /= 2;
	cout << cnt << endl;
	return 0;
}