import java.awt.Point;
import java.awt.Rectangle;
import java.awt.geom.Rectangle2D;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.*;
import java.util.Map.Entry;
import java.util.concurrent.ConcurrentSkipListSet;

public class Main {
	public static final int C =  1000000007;
	static final int CY = 1000000000;
	//static boolean MAP[][];
	static int N;
	static int B;
	static int[][] POS;
	static String S;
	static int X[][];
	static int HA[];
	static long DP[];
	static int ST[][];
	static ConcurrentSkipListSet<Integer> TS;
	static ArrayList<Integer> AL;
	public static void main(String[] args) {
		StringBuilder sb = new StringBuilder();
		BufferedInputStream bs = new BufferedInputStream(System.in);
		Scanner sc = new Scanner(bs);

		N = sc.nextInt();
		long[] a = new long[N];
		long min = Long.MAX_VALUE;
		for (int i =0; i < N; i++) {
			a[i] = sc.nextLong()*1 + sc.nextLong()*4;
			min = Math.min(min, a[i]);
		}
		
		
		long ans = 0;
		
		
		for (int i=0; i < N; i++) {
			long num = a[i] - min;
			if (num %2 != 0) {
				ans = -1;
				break;
			}
			ans += num/2;
		}
		
		System.out.println(ans);
		
		
	}
}