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

using namespace std;

int main(){
    int n;
    cin>> n;

    vector<int> a(n), b(n);
    vector<long long int> wide(n);
    for(int i=0; i<n; i++){
        scanf("%d %d", &a[i], &b[i]);
        wide[i] = a[i]+4*b[i];
    }

    sort(wide.begin(), wide.end());

    bool flag = true;
    for(int i=1; i<n; i++){
        if(wide[i-1]%2!=wide[i]%2){
            flag = false;
        }
    }

    int long long cnt = 0;
    if(flag){
        for(int i=0; i<n-1; i++){
            cnt += (wide[n-1]-wide[i])/2;
        }
        cout<< cnt<< endl;
    }else{
        cout<< "-1"<< endl;
    }

    return 0;
}