#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

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

    vector<int> len(n);
    for(int i=0; i<n; ++i){
        int a, b;
        cin >> a >> b;
        len[i] = a + 4 * b;
    }

    int maxLen = *max_element(len.begin(), len.end());
    int ans = 0;
    for(int i=0; i<n; ++i){
        if((maxLen - len[i]) % 2 != 0){
            ans = -1;
            break;
        }
        ans += (maxLen - len[i]) / 2;
    }
    cout << ans << endl;

    return 0;
}