package yukicoder;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Main {
	public static void main(String[] args) {
		BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
		try {

			int N = Integer.parseInt(read.readLine());
			int i = 0, eo;
			List<Long> list = new ArrayList<>(N);
			String[] box = read.readLine().split(" ");

			long total = 0;
			total = Integer.parseInt(box[0]);
			total += Integer.parseInt(box[1]) * 4;
			list.add(total);

			if (total % 2 == 0) {
				eo = 0;
			} else {
				eo = 1;
			}

			long max = total;
			while (i++ < N - 1) {
				box = read.readLine().split(" ");
				total = Integer.parseInt(box[0]);
				total += Integer.parseInt(box[1]) * 4;
				list.add(total);

				if (total % 2 != eo) {
					break;
				} else {
					if (max < total) {
						max = total;
					}
				}
			}

			if (i < N) {
				System.out.println(-1);
			} else {
				Iterator<Long> it = list.iterator();
				long ans = 0;
				while (it.hasNext()) {
					ans += max - it.next();
				}

				System.out.println(ans / 2);
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}