#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<cstdio>
using namespace std;
int main(){
    long long N,a,b,i,sum[1000],m=0,ans=0;
    bool flag = false;
    cin >> N;
    for(i=0;i<N;i++){
        cin >> a >> b;
        sum[i] = a + b*4;
        m = max(m,sum[i]);
    }
    for(i=0;i<N-1;i++){
        if(sum[i]%2!=sum[i+1]%2){
            cout << -1;
            return 0;
        }    
    }
    for(i=0;i<N;i++){
        ans += (m-sum[i]);
        if((m-sum[i])%2==0) flag = true;
    }
    if(flag)    cout << ans/2;
    else    cout << ans;
    return 0;
}