#include <cstdio>
#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <string>
#include <cstring>
#include <sstream>
#include <algorithm>
#include <functional>
#include <queue>
#include <stack>
#include <cmath>
#include <iomanip>
using namespace std;
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
template<class T> inline T sqr(T x) { return x*x; }
typedef pair<int, int> P;
typedef long long ll;
typedef unsigned long long ull;

#define rep(i,n)  for(int (i) = 0;(i) < (n);(i)++)
#define clr(a) memset((a), 0 ,sizeof(a))
#define mclr(a) memset((a), -1 ,sizeof(a))
#define all(a)  (a).begin(),(a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define sz(a) (sizeof(a))
#define Fill(a,v) fill((int*)a,(int*)(a+(SZ(a)/SZ(*(a)))),v)
bool cheak(int x, int y, int xMax, int yMax){ return x >= 0 && y >= 0 && xMax > x && yMax > y; }
const int dx[4] = { -1, 0, 1, 0 }, dy[4] = { 0, 1, 0, -1 };
const int mod = 1000000007;
const int INF = 2147483647;


int main()
{
	int n;
	cin >> n;
	ll sum[1010];
	ll Max = 0;
	rep(i, n){
		ll a,b;
		cin >> a >> b;
		sum[i] = a + 4 * b;
		Max = max(Max, sum[i]);
	}
	ll ans = 0;
	rep(i, n){
		if ((Max - sum[i]) % 2){
			cout << -1 << endl;
			return 0;
		}
		ans += (Max - sum[i])/2;
	}

	cout << ans << endl;

	return 0;
}