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 C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>

int main(void) {
    long n, i;
    scanf("%ld", &n);
    long a[n + 1], b[n];
    for ( i = 0; i <= n; i++ ) {
        scanf("%ld", &a[i]);
    }
    for ( i = 0; i < n; i++ ) {
        scanf("%ld", &b[i]);
    }

    long ans = 0, remain_a = 0, remain_b = 0;
    /* 1 番目の街から n 番目の街までを考える */
    for ( i = 0; i < n; i++ ) {
        /* (i - 1) 番目の街の残り戦力を使う */
        remain_a = a[i] - remain_b;
        /* 残り戦力で倒しきれていたら i 番目の街の戦力は残る */
        if ( remain_a <= 0 ) {
            ans += a[i];
            remain_b = b[i];
        }
        /* 倒しきれなかったら i 番目の街の戦力を使用する */
        else {
            remain_a -= b[i];
            /* i 番目の街の戦力で倒しきれていたら余りを持ち越す */
            if ( remain_a <= 0 ) {
                ans += a[i];
                remain_b = 0 - remain_a;
            }
            /* i 番目の街の戦力でも倒しきれなかったら余りはなし */
            else {
                ans += (remain_b + b[i]);
                remain_b = 0;
            }
        }
    }
    /* (n + 1) 番目の街は n 番目の街の残り戦力しかない */
    if ( a[n] > remain_b ) {
        ans += remain_b;
    }
    else {
        ans += a[n];
    }
    printf("%ld\n", ans);
    return 0;
}