import java.util.Scanner;

class Main {
    public static void main(String[] args) throws Exception {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] x = new int[n];
        int[] y = new int[n];
        for(int i=0; i<n; i++) {
            x[i] = in.nextInt();
            y[i] = in.nextInt();
        }

        int answer = y[0] - x[0];
        int ans = 0;

        if(answer <= 0) {
            answer = -1;
        }

        for(int i=1; i<n; i++) {
            if(y[i] - x[i] != answer) {
                answer = -1;
            }
        }

        System.out.println(answer);
    }
}