package perfectws;

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		assert 2 <= n && n <= 1000;
		long max = 0;
		long sum = 0;
		boolean odd = false;
		boolean even = false;
		for(int i=0;i<n;i++) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			assert 0 <= a && a <= 10000000 && 0 <= b && b <= 10000000;
			int width = a + b * 4;
			max = Math.max(max, width);
			sum += width;
			if (width % 2 == 0) {
				even = true;
			}else{
				odd = true;
			}
		}
		if (odd && even) {
			System.out.println(-1);
		}else{
			System.out.println((max * n - sum) / 2);
		}

	}
}