#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <utility>
#include <stack>
#include <functional>
#include <queue>
#include <cmath>
#include <set>

#define rep(i,j,k) for(int i=j;i<k;i++)
#define drep(n,m) for(int i=0;i<n;i++)for(int j=0;j<m;j++)
#define Sort(x) sort((x).begin(),(x).end())
#define fi first
#define se second
#define vi vector<int>
#define INF 1000000000
#define MOD 1000000009
#define pb push_back
#define mp make_pair
typedef long long ll;
typedef unsigned long long ull;
int dx[4]={0,1,0,-1},dy[4]={1,0,-1,0};

using namespace std;

ll gcd(ll a,ll b){
    if(a<b){
        int t=a;a=b;b=t;
    }
    int c=a%b;
    while(c!=0){
        a=b;
        b=c;
        c=a%b;
    }
    return b;
}

ll lcm(ll a,ll b){
    return a*b/gcd(a,b);
}

bool isPrime(int x){
    if(x==1)return false;
    if(x==2)return true;
    if(x%2==0)return false;
    for(int i=3;i<=sqrt(x);i+=2){
        if(x%i==0)return false;
    }
    return true;
}

int kaizyo(int x){
    if(x==1)return x;
    else return x*kaizyo(x-1);
}

typedef struct Point{
    int x;
    int y;
} P;

int main() {
    int n;
    cin>>n;
    vector<int> v(n);
    int ma=0;
    bool odd=false,even=false;
    rep(i,0,n){
        int a,b;
        cin>>a>>b;
        v[i]=a+b*4;
        if(v[i]%2==0)even=true;
        else odd=true;
        ma=max(ma,v[i]);
    }
    if(odd&&even){
        cout<<-1<<endl;
        return 0;
    }
    ll ans=0;
    rep(i,0,n){
        ans+=ma-v[i];
    }
    cout<<ans/2<<endl;

    return 0;
}