import java.io.PrintWriter;
import java.util.*;
import java.io.*;
import java.util.function.*;

public class Main {

    public static void main(String[] args) { new Main().stream(); }

    final FastIO      io   = new FastIO();
    final PrintWriter out  = new PrintWriter(System.out);

    void stream(){ solve(); out.flush(); }

    long f(int[] a, int[] b){
        long[] sums = new long[a.length];
        REP(a.length,i-> sums[i] = a[i] + b[i] * 4);
        long oddNum = Arrays.stream(sums).filter(i -> i % 2 == 0).count();
        if(oddNum==0||oddNum==sums.length){
            long max     = Arrays.stream(sums).max().getAsLong();
            long diffSum = Arrays.stream(sums).map(i -> max - i ).sum();
            return diffSum / 2;
        }else{
            return -1;
        }
    }

    void solve() {
        int N = io.Int();
        int[] a = new int[N];
        int[] b = new int[N];
        REP(N, i->{
            a[i] = io.Int();
            b[i] = io.Int();
        });

        System.out.println(f(a,b));

    }
    void FOR(int a, int b, Consumer<Integer>act) { for(int i = a; i < b; ++i) act.accept(i); }
    void REP(int a, Consumer<Integer>act) { FOR(0, a, act); }
}

class FastIO{
    private final InputStream in = System.in;
    private final byte[] buffer  = new byte[1<<12];
    private int   ptr = 0, buffLen = 0;
    private boolean hasNextByte     ()      {
        if(ptr < buffLen) return true;
        ptr = 0;
        try{ buffLen = in.read(buffer); } catch (IOException e){ e.printStackTrace(); }
        return buffLen > 0;
    }
    private int     readByte        ()      { return hasNextByte()? buffer[ptr++] : -1; }
    private boolean isPrintableChar (int c) { return 33 <= c && c <= 126; } //ASCII
    private void    skipUnprintable ()      { while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; }
    private boolean hasNext         ()      { skipUnprintable(); return hasNextByte();}
    private String  next            ()      {
        if(!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)){ sb.appendCodePoint(b); b = readByte(); }
        return sb.toString();
    }
    public String String () { return next(); }
    public char   Char   () { return next().charAt(0);}
    public int    Int    () { return Integer.parseInt(next());}
    public long   Long   () { return Long.parseLong(next());}
    public double Double () { return Double.parseDouble(next());}
}