#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>

#include<map>
#include<set>
#include<string>
#include<algorithm>
#include<functional>
using namespace std;
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define INF 1<<30
#define MP make_pair
#define mp make_pair
#define pb push_back
#define PB push_back
#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define ll long long
#define ull unsigned long long

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n;cin>>n;
	ll s[n];
	REP(i,n){
		ll a,b;cin>>a>>b;
		s[i]=a+b*4;
	}
	ll ma=*max_element(s,s+n);
	ll cnt=0;
	REP(i,n){
		if((ma-s[i])%2==0){
			cnt+=(ma-s[i])/2;
		}else{
			cout<<-1<<endl;return 0;
		}
	}
	cout<<cnt<<endl;
}