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

public class No00000185_Main {

	public static void main(String[] args) throws IOException {

		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		int n = Integer.parseInt(br.readLine());

		String[] strArr = br.readLine().split(" ");
		int x = Integer.parseInt(strArr[0]);
		int y  = Integer.parseInt(strArr[1]);
		int ans = y - x;
		if(ans <= 0) {
			System.out.println(-1);
			return;
		}

		n--;
		boolean flg = true;
		while(n > 0) {
			strArr = br.readLine().split(" ");
			x = Integer.parseInt(strArr[0]);
			y  = Integer.parseInt(strArr[1]);
			if(ans != y - x) {
				flg = false;
				break;
			}
			n--;
		}
		System.out.println(flg ? ans : -1);
	}
}