#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> VI;
typedef vector<VI> VVI;
#define REP(i, n)           for(int(i)=0;(i)<(n);++(i))
#define FOR(i, f, t)        for(int(i)=(f);(i)<(t);(++i))
#define RREP(i, n)          for(int(i)=(n)-1;(i)>=0;--(i))
const int MOD = int(1e9+7);

int N,M,W,H;
int res = 0;
int l[1111];
int f[2];

int main(){
    do { cin.tie(0); ios_base::sync_with_stdio(false); } while(0);
    cin >> N;
    if(N>1000)throw;
    int maxl = 0;
    REP(i,N){
        int a,b;
        cin >> a >> b;
        if(a>10000000||b>10000000)throw;
        int l_ = a+4*b;
        f[l_%2] = 1;
        l[i] = l_;
        maxl = max(maxl, l_);
    }
    if(f[0] && f[1]){
        cout << -1 << endl;
        return 0;
    }
    int res = 0;
    REP(i,N){
        res += (maxl - l[i])/2;
    }

    cout << res << endl;
    return 0;
}