#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <vector>
#include <algorithm>
#include <set>
#include <queue>
#include <map>
#include <climits>
using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define RREP(i,n) for(int i=(int)n-1; i>=0; i--)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();++i)
#define ALL(c) (c).begin(), (c).end()

typedef long long int ll;
typedef pair<int, int> pii;
typedef pair<int, pair<int, int> > pipii;
typedef vector<int> vi;

const int INF = 1e9;
const int MOD = 1e9+7;

int main(void){
	int n;
    cin >> n;
    vi l(n);
    REP(i, n){
        int a, b;
        cin >> a >> b;
        l[i] = a + 4 * b;        
    }
    sort(l.rbegin(), l.rend());
    ll ans = 0;
    REP(i, n){
        if(l[i] % 2 != l[0] % 2){
            cout << "-1" << endl;
            return 0;   
        }
        if(i){
            ans += (l[0] - l[i]);
        }
    }
    cout << ans / 2 << endl;
    return 0;
}