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 (1.14.1) (judge verdict: Accepted)

//go:generate echo "https://atcoder.jp/contests/abc135/tasks/abc135_c"
package main

import (
	"bufio"
	"fmt"
	"io"
	"math/big"
	"os"
	"strconv"
)

var scan = newScanner(os.Stdin)

func solve(As, Bs []int) int {
	sum := 0
	for i := 0; i < len(Bs); i++ {
		if As[i]-Bs[i] > 0 {
			sum += Bs[i]
		} else {
			sum += As[i]
			if As[i+1]-(Bs[i]-As[i]) > 0 {
				sum += Bs[i] - As[i]
				As[i+1] -= Bs[i] - As[i]
			} else {
				sum += As[i+1]
				As[i+1] = 0
			}
		}
	}
	return sum
}

func main() {
	N := scan.Int()
	As := scan.Ints(N + 1)
	Bs := scan.Ints(N)
	fmt.Println(solve(As, Bs))
}

type scanner struct {
	*bufio.Scanner
}

func newScanner(r io.Reader) *scanner {
	s := bufio.NewScanner(r)
	s.Split(bufio.ScanWords)
	s.Buffer(nil, 100000000)
	return &scanner{s}
}

func (s *scanner) String() string {
	s.Scan()
	return s.Text()
}

func (s *scanner) Strings(l int) []string {
	if l == 0 {
		return []string{}
	}
	sl := make([]string, l, l)
	for i := range sl {
		sl[i] = s.String()
	}
	return sl
}

func (s *scanner) Int() int {
	n, _ := strconv.Atoi(s.String())
	return n
}

func (s *scanner) Ints(l int) []int {
	if l == 0 {
		return []int{}
	}
	sl := make([]int, l, l)
	for i := range sl {
		sl[i] = s.Int()
	}
	return sl
}

func (s *scanner) Float64() float64 {
	f, _ := strconv.ParseFloat(s.String(), 64)
	return f
}

func (s *scanner) Float64s(l int) []float64 {
	if l == 0 {
		return []float64{}
	}
	sl := make([]float64, l, l)
	for i := range sl {
		sl[i] = s.Float64()
	}
	return sl
}

func (s *scanner) BigInt() *big.Int {
	n, _ := strconv.ParseInt(s.String(), 10, 64)
	return big.NewInt(n)
}

func (s *scanner) BigInts(l int) []*big.Int {
	if l == 0 {
		return []*big.Int{}
	}
	sl := make([]*big.Int, l, l)
	for i := range sl {
		sl[i] = s.BigInt()
	}
	return sl
}

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

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

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

func ipow(x, n int) int {
	if n == 0 {
		return 1
	}
	a := ipow(x, n>>1)
	a *= a
	if n&1 != 0 {
		a *= x
	}
	return a
}

func isum(X []int) int {
	s := 0
	for _, x := range X {
		s += x
	}
	return s
}

func chmax(x *int, v int) {
	if *x < v {
		*x = v
	}
}

func chmin(x *int, v int) {
	if *x > v {
		*x = v
	}
}

func factor(n int) map[int]int {
	m := map[int]int{}
	for i := 2; i*i <= n; i++ {
		for n%i == 0 {
			n /= i
			m[i]++
		}
	}
	if n != 1 {
		m[n]++
	}
	return m
}

var mod = 1000000007

func setMintMod(x int) {
	mod = x
}

type mint int

func newMint(v int) mint {
	return mint((v%mod + mod) % mod)
}

func (x mint) Add(y mint) mint {
	v := x + y
	if v >= mint(mod) {
		return v - mint(mod)
	}
	return v
}

func (x mint) Sub(y mint) mint {
	v := x + mint(mod) - y
	if v >= mint(mod) {
		return v - mint(mod)
	}
	return v
}

func (x mint) Mul(y mint) mint {
	return (x * y) % mint(mod)
}

func (x mint) pow(n int) mint {
	if n == 0 {
		return mint(1)
	}
	a := x.pow(n >> 1)
	a = a.Mul(a)
	if n&1 != 0 {
		a = a.Mul(x)
	}
	return a
}

func (x mint) Inverse() mint {
	return x.pow(mod - 2)
}

func (x mint) Div(y mint) mint {
	return x * y.Inverse()
}

func (x mint) Neg() mint {
	return newMint(int(-x))
}

type factorial struct {
	fact        []mint
	factInverse []mint
}

func newFactorial(n int) *factorial {
	f := new(factorial)
	f.init(n)
	return f
}

func (f *factorial) init(n int) {
	fact := make([]mint, n+1, n+1)
	fact[0] = newMint(1)
	for i := 1; i <= n; i++ {
		fact[i] = fact[i-1].Mul(newMint(i))
	}

	inv := make([]mint, n+1, n+1)
	inv[n] = fact[n].Inverse()
	for i := n; i >= 1; i-- {
		inv[i-1] = inv[i].Mul(newMint(i))
	}

	f.fact = fact
	f.factInverse = inv
}

func (f *factorial) Get(n int) mint {
	return f.fact[n]
}

func (f *factorial) GetInverse(n int) mint {
	return f.factInverse[n]
}

func (f *factorial) Permutation(n, k int) mint {
	if k < 0 || n < k {
		return 0
	}
	return f.fact[n].Mul(f.factInverse[n-k])
}

func (f *factorial) Combination(n, k int) mint {
	if k < 0 || n < k {
		return 0
	}
	return f.fact[n].Mul(f.factInverse[k]).Mul(f.factInverse[n-k])
}

type sieve struct {
	n          int
	primes     []int
	maxFactors []int
}

func newSieve(n int) *sieve {
	s := &sieve{
		n:          n,
		primes:     []int{},
		maxFactors: make([]int, n+1, n+1),
	}
	s.init()
	return s
}

func (s *sieve) init() {
	s.maxFactors[0] = -1
	s.maxFactors[1] = -1
	for i := 2; i <= s.n; i++ {
		if s.maxFactors[i] != 0 {
			continue
		}
		s.primes = append(s.primes, i)
		s.maxFactors[i] = i
		for j := i * i; j <= s.n; j += i {
			if s.maxFactors[j] == 0 {
				s.maxFactors[j] = i
			}
		}
	}
}

func (s *sieve) IsPrime(x int) bool {
	return s.maxFactors[x] == x
}

func (s *sieve) Factor(x int) map[int]int {
	m := map[int]int{}
	for x != 1 {
		m[s.maxFactors[x]]++
		x /= s.maxFactors[x]
	}
	return m
}