#include<iostream>

using namespace std;

int main(){
    
long long N;
long long a[1000] ,b[1000];

cin >> N;
for(int i=0; i<N; i++)  cin >> a[i] >> b[i];

int maxLength = a[0] + 4*b[0];
int maxline =0;

for(int i=0; i<N; i++){
    if( a[i] + 4*b[i] > maxLength){
        maxLength = a[i] + 4*b[i];
        maxline = i;
    }
}

int surplus = maxLength %2;
int spaceCounter = 0;

for(int i=0; i<N; i++){
    if(i==maxline) continue;
    
    if( (a[i] + 4*b[i])%2 != surplus ){
        cout << -1 << endl;
        return 0;
    }
    else spaceCounter += (maxLength - (a[i] + 4*b[i])) / 2; 
}

cout << spaceCounter << endl;

}