#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()

#define int long long

signed main(){
    int n;cin>>n;
    vector<int> data(n);
    int myMax=0;
    REP(i,n){
        int a,b;cin>>a>>b;
        data[i]=a+4*b;
        myMax=max(myMax,data[i]);
    }
    sort(ALL(data),greater<int>());
    int res=0;
    FOR(i,1,n){
        int t=myMax-data[i];
        if(t%2!=0){
            res=-1;
            cout<<-1<<"\n";
            break;
        }else{
            res+=(t/2);
        }
    }
    if(res!=-1){
        cout<<res<<"\n";
    }
    return 0;
}