import java.io.*;
import java.util.*;
import java.math.*;

class Main178 {
	
	public static void out (Object out) {
		System.out.println(out);
	}
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		int n = Integer.parseInt(br.readLine());
		int[] nums = new int[n];
		String[] line = br.readLine().split(" ");
		int a = Integer.parseInt(line[0]);
		nums[0] = a + Integer.parseInt(line[1]) * 4;
		int max = nums[0];
		
		boolean isImpossible = false;
		for (int i = 1; i < n; i++) {
			line = br.readLine().split(" ");
			int a2 = Integer.parseInt(line[0]);
			if (a % 2 != a2 % 2) {
				isImpossible = true;
				break;
			}
			nums[i] = a2 + Integer.parseInt(line[1]) * 4;
			max = Math.max(max , nums[i]);
		}
		//out("max = " + max);
		int ans = 0;
		for (int m : nums) ans += (max - m);
		
		out(isImpossible ? -1 : ans / 2);
		
	}
}