#define _USE_MATH_DEFINES
#include<iostream>
#include<cstdio>
#include<algorithm>
#include<climits>
#include<string>
#include<vector>
#include<list>
#include<map>
#include<set>
#include<cmath>
#include<queue>
#include<cstring>
#include<stack>
using namespace std;
typedef long long ll;

int main(){
	int N;
	cin>>N;
	int size[1000], ma = 0;
	for(int i=0;i<N;i++){
		int A,B;
		cin>>A>>B;
		ma = max(ma,size[i] = A+B*4);
	}
	int ans = 0;
	for(int i=0;i<N;i++){
		if((ma-size[i])&1!=0){
			ans = -1; break;
		}
		ans += (ma-size[i])/2;
	}
	printf("%d\n",ans);
	return 0;
}