package main

import (
	"fmt"
	"math"
)

var (
	N int
	a []int
	b []int
)

func resolve() float64 {
	ret := float64(0)
	maxLen := float64(a[0]*1 + b[0]*4)
	rest := a[0] % 2
	for i := 0; i < N; i++ {
		tmp := float64(a[i]*1 + b[i]*4)
		if a[i]%2 != rest {
			return -1
		}
		if maxLen > tmp {
			ret += math.Abs(maxLen-tmp) / 2
			continue
		}
		ret += math.Abs(maxLen-tmp) / 2 * float64(i)
		maxLen = tmp
	}
	return ret
}

func main() {
	fmt.Scanf("%d\n", &N)
	a = make([]int, N)
	b = make([]int, N)
	for i := 0; i < N; i++ {
		fmt.Scanf("%d %d\n", &a[i], &b[i])
	}
	fmt.Println(resolve())
}