import java.util.Scanner;

public class yukicoder_185 {
	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		
		int n = stdIn.nextInt();
		
		int[] x = new int[n];
		int[] y = new int[n];
			
		for (int i = 0; i < n; i++) {
			x[i] = stdIn.nextInt();
			y[i] = stdIn.nextInt();
		}
		
		int answer = -1;
		int diff = y[0] - x[0];
		
		if (diff >= 1) {
			for (int i = 1; i < n; i++) {
				if (x[i] + diff == y[i]) {
					answer = diff;
				} else {
					answer = -1;
					break;
				}
			}
		}
		System.out.println(answer);
	}
}