#include<bits/stdc++.h>
using namespace std;
#define int long long
#define vi vector<int>
#define vvi vector<vi>
#define vs vector<string>
#define pb push_back
#define P pair<int,int>
#define vp vector<P>
#define PP pair<P,int>
#define vpp vector<PP>
#define fi first
#define se second
#define INF (1<<30)
#define MOD 1000000007
#define REP(i,n) for(int i=0;i<n;i++)
#define REPR(i,n) for(int i=n;i>=0;i--)
#define FOR(i,m,n) for(int i=m;i<n;i++)
#define all(x) (x).begin(),(x).end()
bool comp(const P &a,const P &b){
    return a.fi+a.se*4>b.fi+b.se*4;
}
signed main(){
    int n;
    cin>>n;
    vp d(n);
    REP(i,n)cin>>d[i].fi>>d[i].se;
    sort(all(d),comp);
    bool f=false;
    f=(d[0].fi+d[0].se*4)%2;
    int ans=0;
    REP(i,n){
        if((d[i].fi+d[i].se*4)%2!=f){
            cout<<"-1"<<endl;
            return 0;
        }
        ans+=(d[0].fi+d[0].se*4-d[i].fi-d[i].se*4)/2;
    }
    cout<<ans<<endl;
    return 0;
}