import java.util.Scanner;

class Test {
	public static void main(String[] args) throws Exception {
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();

		int[] a = new int[n];
		int[] b = new int[n];
		int[] w = new int[n];

		for(int i=0; i<n; i++) {
			a[i] = in.nextInt();
			b[i] = in.nextInt();
			w[i] = a[i] + b[i] * 4;
		}

		int temp = 0;
		for(int j=1; j<n; j++) {
			if(w[j] > w[j-1]) {
				temp = w[j];
			}
		}

		int ans = 0;
		for(int k=0; k<n; k++) {
			ans = temp - w[k];
		}

		if(ans % 2 != 0) {
			System.out.println(-1);
		} else {
			System.out.println(ans / 2);
		}

	}
}