import java.io.*;
import java.util.*;
import java.math.*;

class Main185 {
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		int ans = -1;
		boolean flg = false;
		for (int i = 0; i < n; i++) {
			if (flg) continue;
			String[] str = br.readLine().split(" ");
			int sub = Integer.parseInt(str[1]) - Integer.parseInt(str[0]);
			if (sub < 1) {
				flg = true;
				continue;
			}
			if (i == 0) {
				ans = sub;
			} else if (ans != sub) {
				flg = true;
			}
		}
		System.out.println(flg ? -1 : ans);
	}
}