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: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"sort"
	"strconv"
)

func main() {
	n := readi()
	a := make([]int64, n+1)
	for i := 0; i < n+1; i++ {
		a[i] = readi64()
	}
	b := make([]int64, n)
	for i := 0; i < n; i++ {
		b[i] = readi64()
	}

	var ans int64
	for i := 0; i < n; i++ {
		if a[i] <= b[i] {
			ans = ans + a[i]
			tmp := (b[i] - a[i])
			if a[i]+1 <= tmp {
				ans = ans + a[i+1]
				a[i+1] = 0
			} else {
				ans = ans + tmp
				a[i+1] = a[i+1] - tmp
			}
		} else {
			ans = ans + b[i]
		}
	}

	fmt.Println(ans)
}

/*-------------------utilities-------------------*/

func abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}

func min(x, y int) int {
	if x < y {
		return x
	}
	return y
}

func max(x, y int) int {
	if x > y {
		return x
	}
	return y
}

func chmin(x *int, y int) bool {
	if *x > y {
		*x = y
		return true
	}
	return false
}

func chmax(x *int, y int) bool {
	if *x < y {
		*x = y
		return true
	}
	return false
}

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

func lcm(a, b int) int {
	return (a * b) / gcd(a, b)
}

func comb(n, k int) int {
	if n < k {
		return 0
	}
	if n < 0 || k < 0 {
		return 0
	}
	return fac[n] * (finv[k] * finv[n-k] % MOD) % MOD
}

func permutations(arr []int) [][]int {
	var helper func([]int, int)
	res := [][]int{}

	helper = func(arr []int, n int) {
		if n == 1 {
			tmp := make([]int, len(arr))
			copy(tmp, arr)
			res = append(res, tmp)
		} else {
			for i := 0; i < n; i++ {
				helper(arr, n-1)
				if n%2 == 1 {
					tmp := arr[i]
					arr[i] = arr[n-1]
					arr[n-1] = tmp
				} else {
					tmp := arr[0]
					arr[0] = arr[n-1]
					arr[n-1] = tmp
				}
			}
		}
	}
	helper(arr, len(arr))
	return res
}

func isPrime(n int) bool {
	if n == 1 {
		return false
	}
	for i := 2; i*i <= n; i++ {
		if n%i == 0 {
			return false
		}
	}
	return true
}

func divisor(n int) (res []int) {
	for i := 1; i*i <= n; i++ {
		if n%i == 0 {
			res = append(res, i)
			if i*i != n {
				res = append(res, n/i)
			}
		}
	}
	sort.Ints(res)
	return
}

/*-------------------init-------------------*/

const (
	INF = 1000000000000000000
	MOD = 1e9 + 7

	COMB_MAX = 510000
)

var (
	readString func() string
)

var (
	fac  = [COMB_MAX]int{}
	finv = [COMB_MAX]int{}
	inv  = [COMB_MAX]int{}
)

func init() {
	readString = newReadString(os.Stdin)
}

func combInit() {
	fac[0] = 1
	fac[1] = 1
	finv[0] = 1
	finv[1] = 1
	inv[1] = 1
	for i := 2; i < COMB_MAX; i++ {
		fac[i] = fac[i-1] * i % MOD
		inv[i] = MOD - inv[MOD%i]*(MOD/i)%MOD
		finv[i] = finv[i-1] * inv[i] % MOD
	}
}

/*-------------------inputs-------------------*/

func readi() int {
	return int(readi64())
}

func readi64() int64 {
	i, err := strconv.ParseInt(readString(), 0, 64)
	if err != nil {
		panic(err.Error())
	}
	return i
}

func readf() float64 {
	f, err := strconv.ParseFloat(readString(), 64)
	if err != nil {
		panic(err.Error())
	}
	return f
}

func newReadString(ior io.Reader) func() string {
	r := bufio.NewScanner(ior)
	r.Buffer(make([]byte, 1024), int(1e+11))
	r.Split(bufio.ScanWords)

	return func() string {
		if !r.Scan() {
			panic("Scan failed")
		}
		return r.Text()
	}
}