import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
import java.math.*;
 
public class Main {
	public static void main(String[] args)throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringBuilder sb = new StringBuilder();
		int n = Integer.parseInt(br.readLine());
		int[] sum = new int[n];
		int max = 0;
		for(int i = 0; i < n; i++){
			String[] str = br.readLine().split(" ");
			sum[i] = Integer.parseInt(str[0]);
			sum[i] += 4*Integer.parseInt(str[1]);
			max = Math.max(max,sum[i]);
		}
		int cnt = 0;
		for(int i = 0; i < n; i++){
			int k = max - sum[i];
			if(k%2!=0){
				sb.append(-1);
				System.out.println(sb);
				return;
			}
			cnt += k/2;
		}
		sb.append(cnt);
		System.out.println(sb);
	}
}