#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
    int N;
    int m_len = 0;
    cin >> N;
    vector<int> len(N);
    for(int i = 0; i < N; i++){
        int a, b;
        cin >> a >> b;
        m_len = max(m_len, a + b*4);
        len[i] = a + b*4;
    }
    int ans = 0;
    for(int i = 0; i < N; i++){
        int diff = m_len - len[i];
        if(diff % 2 == 1){
            ans = -1;
            break;
        }
        ans += diff/2;
    }
    cout << ans << endl;
    return 0;
}