package main

import (
	"fmt"
)

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

func abs(x int64) int64 {
	if x < 0 {
		return x * -1
	}
	return x
}

func resolve() int64 {
	ret := int64(0)
	maxLen := int64(a[0]*1 + b[0]*4)
	rest := a[0] % 2
	for i := 0; i < N; i++ {
		tmp := int64(a[i]*1 + b[i]*4)
		if a[i]%2 != rest {
			return -1
		}
		if maxLen > tmp {
			ret += abs(maxLen-tmp) / 2
			continue
		}
		ret += abs(maxLen-tmp) / 2 * int64(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())
}