#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>
#include <limits>

#define N 1000
#define MOD 1000000007
#define pe(str) return cout<<(str)<<endl,0
#define px(str) {cout<<str<<endl;exit(0);}
#define re(i,n) for(int i=0;i<(n);i++)
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define fe(i,n,f) for_each(i,n,f)
#define bw(a,b,c) (((a)<=(b))&&((b)<=(c)))
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pii;

int main(void){
	int a,b;
	int n;
	int g[2]={0,0};
	cin>>n;
	int k[n];
	re(i,n){
		cin>>a>>b;
		k[i]=a+4*b;
		g[k[i]%2]++;
	}
	if(g[0]>0&&g[1]>0)pe(-1);
	if(g[1]) re(i,n) k[i]--;
	int d=*max_element(k,k+n);
	int s=0;
	re(i,n) s+=(d-k[i])/2.0;

	cout<<s<<endl;

	return 0;
}