import java.util.Scanner;

class no178{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		long n=stdIn.nextLong(),a,b,s[]=new long[1000],m=0,t=0,ans=0;
		int i;
		for(i=0;i<n;i++) {
			a=stdIn.nextLong();
			b=stdIn.nextLong();
			s[i]+=a+4*b;
			if(m<s[i]) m=s[i];
		}
		for(i=0;i<n;i++) {
			if((m-s[i])%2!=0) t++;
			ans+=m-s[i];
		}
		System.out.print(t!=0?"-1":ans/2);
		
	}
}