import java.io.BufferedReader;
import java.io.InputStreamReader;

public class No105 {

    public static void main(String[] args) {
        try {
            BufferedReader br = new BufferedReader(new InputStreamReader(
                    System.in));
            int N = Integer.parseInt(br.readLine());

            String[] XY;
            int X, Y, ans = 0, beforeAns = 0;

            for (int i = 0; i < N; i++) {
                XY = br.readLine().split(" ");
                X = Integer.parseInt(XY[0]);
                Y = Integer.parseInt(XY[1]);

                ans = Y - X;
                if (i == 0) {
                    beforeAns = ans;
                }

                if (ans < 1 || ans != beforeAns) {
                    ans = -1;
                    break;
                }
                beforeAns = ans;
            }

            System.out.println(ans);
        } catch (Exception e) {
            System.err.println("Error:" + e.getMessage());
        }
    }

}