#include <iostream>
#include <string>
#include <queue>
#include <stack>
#include <algorithm>
#include <list>
#include <vector>
#include <complex>
#include <utility>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <bitset>
#include <ctime>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <cassert>
#include <cstddef>
#include <iomanip>
#include <numeric>

#define REP(i,n) for(long (i)=0;(i)<(n);(i)++)
#define FOR(i,a,b) for(long (i)=(a);(i)<(b);(i)++)
#define RREP(i,a) for(long (i)=(a)-1;(i)>=0;(i)--)
#define FORR(i,a,b) for(long (i)=(a)-1;(i)>=(b);(i)--)
#define MOD 1000000007
#define PI acos(-1.0)
#define DEBUG(C) cout<<C<<endl;
#define PII pair<int,int>
#define PLL pair<long,long>
#define ALL(a) (a).begin(),(a).end()
#define SORT(a) sort((a).begin(),(a).end())

typedef long long LL;
typedef unsigned long long ULL;

using namespace std;

int main(void){
	int N; cin>>N;
	LL ans=0;
	long a,b;
	vector<LL> v(N);
	REP(i,N){
		cin>>a>>b;
		v[i]=a+(4*b);
	}
	SORT(v);
	REP(i,N-1){
		if((v[i]&1)!=(v[N-1]&1)){
			cout<<-1<<endl;
			return 0;
		}
		ans+=v[N-1]-v[i];
	}
	cout<<ans/2<<endl;
}