import java.util.Scanner;

public class Test {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		long n = sc.nextLong(), sum = 0, max = 0, t;
		boolean a = false, b = false;
		for(int i = 0; i < n; i++){
			t = sc.nextLong() + sc.nextLong() * 4;
			if(t % 2 == 0){
				a = true;
			} else {
				b = true;
			}
			max = Math.max(max, t);
			sum += t;
		}
		if(a && b){
			System.out.println(-1);
			return;
		}
		System.out.println((max*n - sum)/2);
	}
}