Problem:
Score : 300 points

Problem Statement

There are N+1 towns. The i-th town is being attacked by A_i monsters.

We have N heroes. The i-th hero can defeat monsters attacking the i-th or (i+1)-th town, for a total of at most B_i monsters.

What is the maximum total number of monsters the heroes can cooperate to defeat?

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^9

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_{N+1}
B_1 B_2 ... B_N

Output

Print the maximum total number of monsters the heroes can defeat.

Sample Input 1

2
3 5 2
4 5

Sample Output 1

9

If the heroes choose the monsters to defeat as follows, they can defeat nine monsters in total, which is the maximum result.

The first hero defeats two monsters attacking the first town and two monsters attacking the second town.

The second hero defeats three monsters attacking the second town and two monsters attacking the third town.

Sample Input 2

3
5 6 3 8
5 100 8

Sample Output 2

22

Sample Input 3

2
100 1 1
1 100

Sample Output 3

3

Solution in Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"sort"
	"strconv"
	"strings"
)

func main() {
	io := NewIo()
	defer io.Flush()

	n := io.Int()
	aa := make([]int, n+1)
	for i := 0; i < n+1; i++ {
		aa[i] = io.Int()
	}

	bb := make([]int, n)
	for i := 0; i < n; i++ {
		bb[i] = io.Int()
	}

	ans := Min(aa[0], bb[0])
	bb[0] -= Min(aa[0], bb[0])

	for i := 1; i < n; i++ {
		tmp := Min(bb[i-1], aa[i])
		Log("tmp", tmp)
		aa[i] -= tmp
		ans += tmp
		tmp = Min(aa[i], bb[i])
		bb[i] -= tmp
		ans += tmp
		Log("tmp", tmp)
	}
	ans += Min(bb[n-1], aa[n])

	fmt.Println(ans)
}

type Io struct {
	reader    *bufio.Reader
	writer    *bufio.Writer
	tokens    []string
	nextToken int
}

func NewIo() *Io {
	return &Io{
		reader: bufio.NewReader(os.Stdin),
		writer: bufio.NewWriter(os.Stdout),
	}
}

func (io *Io) Flush() {
	err := io.writer.Flush()
	if err != nil {
		panic(err)
	}
}

func (io *Io) NextLine() string {
	var buffer []byte
	for {
		line, isPrefix, err := io.reader.ReadLine()
		if err != nil {
			panic(err)
		}
		buffer = append(buffer, line...)
		if !isPrefix {
			break
		}
	}
	return string(buffer)
}

func (io *Io) Next() string {
	for io.nextToken >= len(io.tokens) {
		line := io.NextLine()
		io.tokens = strings.Fields(line)
		io.nextToken = 0
	}
	r := io.tokens[io.nextToken]
	io.nextToken++
	return r
}

func (io *Io) Int() int {
	i, err := strconv.Atoi(io.Next())
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) Float() float64 {
	i, err := strconv.ParseFloat(io.Next(), 64)
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) PrintLn(a ...interface{}) {
	fmt.Fprintln(io.writer, a...)
}

func (io *Io) Printf(format string, a ...interface{}) {
	fmt.Fprintf(io.writer, format, a...)
}

func (io *Io) PrintIntLn(a []int) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.PrintLn(b...)
}

func (io *Io) PrintStringLn(a []string) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.PrintLn(b...)
}

func Log(name string, value interface{}) {
	fmt.Fprintf(os.Stderr, "%s=%+v\n", name, value)
}

func Abs(a int) int {
	if a >= 0 {
		return a
	} else {
		return -a
	}
}

func Pow(p, q int) int {
	return int(math.Pow(float64(p), float64(q)))
}

func Min(nums ...int) int {
	if len(nums) == 0 {
		panic("funciton min() requires at least one argument.")
	}
	res := nums[0]
	for i := 0; i < len(nums); i++ {
		res = int(math.Min(float64(res), float64(nums[i])))
	}
	return res
}

func Max(nums ...int) int {
	if len(nums) == 0 {
		panic("funciton max() requires at least one argument.")
	}
	res := nums[0]
	for i := 0; i < len(nums); i++ {
		res = int(math.Max(float64(res), float64(nums[i])))
	}
	return res
}

func SortAsc(a []int) []int {
	sort.Ints(a)
	return a
}

func SortDesc(a []int) []int {
	sort.Sort(sort.Reverse(sort.IntSlice(a)))
	return a
}

func GCD(a, b int) int {
	if b == 0 {
		return a
	}
	return GCD(b, a%b)
}

func LCM(a, b int) int {
	if a == 0 && b == 0 {
		return 0
	}
	return a * b / GCD(a, b)
}

func CumSum(nums []int) []int {
	sums := []int{0}

	for i := 0; i < len(nums); i++ {
		sums = append(sums, sums[i]+nums[i])
	}
	return sums
}

func BisectLeft(nums []int, left, right int, target int) int {
	if left == right {
		if nums[left] == target {
			return left
		} else if nums[left] < target {
			return left + 1
		} else {
			return left
		}
	}

	mid := (left + right) / 2
	if nums[mid] == target {
		return mid
	}

	if nums[mid] > target {
		return BisectLeft(nums, left, mid, target)
	} else if nums[mid] < target {
		return BisectLeft(nums, mid+1, right, target)
	}
	return 0
}

func Permutations(nums []int) [][]int {
	n := len(nums)
	if n == 0 {
		return [][]int{}
	}
	ans := [][]int{}

	for _, n := range nums {
		rest := remove(nums, n)
		lists := Permutations(rest)
		if len(lists) == 0 {
			lists = [][]int{nil}
		}

		for i := 0; i < len(lists); i++ {
			lists[i] = append(lists[i], n)
			ans = append(ans, lists[i])
		}
	}
	return ans
}

func remove(nums []int, target int) []int {
	result := []int{}
	for i := 0; i < len(nums); i++ {
		if nums[i] != target {
			result = append(result, nums[i])
		}
	}
	return result
}

func PowerSet(nums []int) [][]int {
	size := Pow(2, len(nums))
	result := make([][]int, size)

	idx := 0
	result[idx] = []int{}
	idx++

	for _, n := range nums {
		max := idx
		for i := 0; i < max; i++ {
			result[idx] = copyAndAppend(result[i], n)
			idx++
		}
	}

	return result
}

func copyAndAppend(nums []int, n int) []int {
	dst := make([]int, len(nums)+1)
	copy(dst, nums)
	dst[len(nums)] = n
	return dst
}

func Combination(nums []int, k int) [][]int {
	size := len(nums)
	result := make([][]int, CombinationCount(len(nums), k))
	bi := (1 << uint(k)) - 1
	max := 1 << uint(size)
	idx := 0

	for bi < max {
		flags := bi
		s := []int{}
		for _, n := range nums {
			if flags%2 != 0 {
				s = append(s, n)
			}
			flags /= 2
		}
		result[idx] = s
		idx++
		bi = Combination2NextIndex(bi)
	}
	return result
}

func Combination2NextIndex(n int) int {
	smallest := n & -n
	ripple := n + smallest
	newSmallest := ripple & -ripple
	ones := ((newSmallest / smallest) >> 1) - 1
	return ripple | ones
}

func CombinationCount(n, m int) int {
	return Fact(n) / (Fact(n-m) * Fact(m))
}

func Fact(n int) int {
	if n == 0 {
		return 1
	} else {
		return n * Fact(n-1)
	}
}

func Divisors(n int) []int {
	divisors := map[int]bool{}
	for i := 1; i < int(math.Sqrt(float64(n)))+1; i++ {
		if n%i == 0 {
			divisors[i] = true
			//divisors = append(divisors, i)
			if i != n {
				divisors[n/i] = true
				//divisors = append(divisors, n/i)
			}
		}

	}
	ans := []int{}
	for k := range divisors {
		ans = append(ans, k)
	}
	return ans
}