import java.util.*;
import java.io.*;
import static java.util.Arrays.*;
import static java.lang.Math.*;

public class No178 {
    
    static final Scanner in = new Scanner(System.in);
    static final PrintWriter out = new PrintWriter(System.out,false);

    static void solve() {
        int n = in.nextInt();
        int[] a = new int[n];
        int max = 0;
        for (int i=0; i<n; i++) {
            a[i] = in.nextInt()+in.nextInt()*4;
            max = max(max,a[i]);
        }
        int cnt = 0;
        for (int i=0; i<n; i++) {
            if ((max - a[i]) %2 == 1) {
                out.println("-1");
                return ;
            }
            cnt += (max - a[i])/2;
        }
        out.println(cnt);
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();

        solve();
        out.flush();

        long end = System.currentTimeMillis();
        //trace(end-start + "ms");
        in.close();
        out.close();
    }

    static void trace(Object... o) { System.out.println(deepToString(o));}
}