import java.util.Scanner;


public class Main_yukicoder178 {

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

        int n = sc.nextInt();
        long[] s = new long[n];
        long max = 0;
        boolean flag = true;
        for (int i = 0; i < n; i++) {
        	int a = sc.nextInt();
        	int b = sc.nextInt();
        	s[i] = (long)a + b * 4;
        	max = Math.max(max, s[i]);
        	if (i > 0 && s[i] % 2 != s[i - 1] % 2) {
        		flag = false;
        	}
        }

        if (flag) {
        	long ret = 0;
        	for (int i = 0; i < n; i++) {
        		ret += (max - s[i]) / 2;
        	}
        	
        	System.out.println(ret);
        } else {
        	System.out.println("-1");
        }
        
        sc.close();
    }
}