#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>

#define ALL(x) (x).begin(),(x).end()
#define RALL(x) (x).rbegin(),(x).rend()
#define REP(i,a,n) for(int i = (a);i < (n);i++)
#define PI 3.14159265359
#define MOD 1000000007

using namespace std;

int main(){
    long long n,ans = 0;
    cin >> n;
    vector<long long> s;
    REP(i,0,n){
        long long a,b,space;
        cin >> a >> b;
        space = a + 4*b;
        s.push_back(space);
    }
    sort(RALL(s));
    REP(i,1,n){
        if((s[0] - s[i])%2 != 0){
            ans = -1;
            break;
        }
        else {
            ans += (s[0] - s[i])/2;
        }
    }
    cout << ans << endl;
    return 0;
}