#include <iostream>

const int N = 1010;
int a[N], b[N];
using namespace std;
int w[N];

int main() {
int n;
cin >> n;
int wm =0;
for(int i = 0; i < n; ++i) {cin >> a[i] >>b[i]; w[i] = a[i]+4*b[i];wm=max(wm,w[i]);}
int cnt =0;
for(int i = 0;i<n;++i) {
if((wm-w[i])%2) {
cout <<-1<<endl;
return 0;
}
cnt+=(wm-w[i])/2;
}
cout<<cnt<<endl;

}