#define _USE_MATH_DEFINES	//M_PI(π),M_SQRT2(√2)
#include <iostream>			//std::cout, std::cin
#include <string>			//std::string
#include <vector>			//std::vector
#include <valarray>			//std::valarray
#include <algorithm>		//std::sort
#include <time.h>			//localtime_s
#include <cstdlib>			//abs
#include <cmath>			//abs,std::pow,sqrt,sin,cos,round,floor,ceil
#include <fstream>			//std::ifstream
#include <iomanip>			//std::setprecision
#include <random>			//std::random(C++11)
#include <numeric>			//std::accumulate

int main(void) {

	//test用
	//std::ifstream in("test.txt");
	//std::cin.rdbuf(in.rdbuf());

	//const unsigned int MODULO = 1000000007;	//MODULO 10^9+7用

	int N, a, b;
	std::cin >> N;
	std::vector<double> L(N);

	bool type;
	bool flg = true;
	double max = 0, ans = 0;

	std::cin >> a >> b;
	type = a % 2;
	max = L[0] = a + b * 4;

	for (int i = 1; i < N; i++) {
		std::cin >> a >> b;
		if (a % 2 != type) {
			flg = false;
			break;
		}
		L[i] = a + b * 4;
		if (max <= L[i]) {
			max = L[i];
		}
	}

	if (flg) {
		for (int i = 0; i < N; i++) {
			ans += (max - L[i]) / 2;
		}
	}

	if (flg) {
		std::cout << std::setprecision(20) << ans << std::endl;
	}
	else {
		std::cout << -1 << std::endl;
	}
}