#include<iostream>
#include<sstream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<numeric>
#include<functional>
#include<complex>

using namespace std;
#define BET(a,b,c) ((a)<=(b)&&(b)<(c))
#define FOR(i,n) for(int i=0,i##_end=(int(n));i<i##_end;i++)
#define SZ(x) (int)(x.size())
#define ALL(x) (x).begin(),(x).end()
#define MP make_pair
#define FOR_EACH(it,v) for(__typeof(v.begin()) it=v.begin(),it_end=v.end() ; it != it_end ; it++)
typedef vector<int> VI;
typedef vector<VI> VVI;

int main()
{
    int N;
    cin>>N;
    VI w(N);
    FOR(i,N){
        int a,b;
        scanf("%d%d",&a,&b);
        w[i] = a + 4 * b;
    }
    int maxW = 0;
    FOR(i,N) maxW = max(maxW, w[i]);
    long long ans = 0 ;
    FOR(i,N){
        int need = maxW - w[i];
        if(need % 2){
            puts("-1");
            return 0;
        }
        ans += need / 2;
    }
    cout<<ans<<endl;
    return 0;
}