package m.shin;
import java.util.Scanner;
public class Con185 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int N = s.nextInt() , d = 0;
		int[] X = new int[N];
		int[] Y = new int[N];
		for(int i = 0;i < N;i++){
			X[i] = s.nextInt();
			Y[i] = s.nextInt();
			if(i >= 1){
				if(d != Y[i] - X[i] || Y[i] - X[i] <= 0){
					d = -1;
				}
			}else{
				if(Y[i] - X[i] <= 0){
					d = -1;
				}else{
					d = Y[i] - X[i];
				}
			}
		}
		s.close();

		System.out.println(d);
	}

}