import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

import static java.lang.System.in;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        int N = Integer.parseInt(reader.readLine());
        String[] inputs;
        long max = Long.MIN_VALUE;
        long[] widths = new long[N];
        for (int i = 0; i < N; i++) {
            inputs = reader.readLine().split(" ");
            long a = Long.parseLong(inputs[0]);
            long b = Long.parseLong(inputs[1]);
            widths[i] = a + 4 * b;
            max = Math.max(max, a + 4 * b);
        }
        long numOfSpaces = 0;
        boolean isPerfect = true;
        for (int i = 0; i < N; i++) {
            if ((max - widths[i]) % 2 != 0) {
                isPerfect = false;
                break;
            } else {
                numOfSpaces += (max - widths[i]) / 2;

            }

        }

        if (isPerfect) {
            System.out.println(numOfSpaces);
        } else {
            System.out.println(-1);
        }

    }
}