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

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 answer = -1;
        for (int i = 0; i < N; i++) {
            inputs = reader.readLine().split(" ");
            long x = Long.parseLong(inputs[0]);
            long y = Long.parseLong(inputs[1]);
            if (y - x > 0) {
                if (answer != -1 && answer != y - x) {
                    answer = -1;
                    break;
                } else {
                    answer = y-x;
                }
            } else {
                answer=-1;
                break;
            }
        }
        System.out.println(answer);
    }
}