#include<iostream>
#include<cmath>
#include<vector>
#include<string>
#include<set>
#include<iomanip>
#include<algorithm>
using namespace std;
int main() {
    int N; cin >> N;
    int a, b; 
    vector<int> v;
    int maxV = 0;
    for(int i=0; i<N; i++){
        cin >> a >> b;
        v.push_back(a + b * 4);
        maxV = max(maxV, v[i]);
    }
    int ans = 0;
    bool f = true;
    if(maxV < 2){
        f = false;
    }else{
        for(int i=0; i<N; i++){
            ans += (maxV - v[i]) / 2;
        }
    }
    if(f) cout << ans << endl;
    else cout << -1 << endl;
    return 0;
}