fun main(args: Array<String>) {
    val heroCount = readLine()!!.toInt()
    val monsterCountParTown = readLine()!!.split(" ").map { it.toInt() }
    val maxKillCountParHero = readLine()!!.split(" ").map { it.toInt() }

    print(abc135c(heroCount, monsterCountParTown, maxKillCountParHero))
}

fun abc135c(heroCount: Int, monsterCountParTown: List<Int>, maxKillCountParHero: List<Int>): Long {
    var ans:Long = 0
    val monsterCount = mutableMapOf<Int, Int>()
    for(i in monsterCountParTown.indices){
        monsterCount[i] = monsterCountParTown[i]
    }

    //協力が出来ていない？
    // 必ず前の街を全力で潰す。二つ目の全力にするパターンもある？
    for (i in 1..heroCount) {
        val target: Pair<Int, Int> = Pair(monsterCount[i - 1]!!, monsterCount[i]!!)
        val attackResult = attackMonster(target, maxKillCountParHero[i - 1])
        ans += attackResult.first
        monsterCount[i - 1] = attackResult.second
        monsterCount[i] = attackResult.third
    }
    return ans
}

fun attackMonster(target: Pair<Int, Int>, maxKillCount: Int): Triple<Long, Int, Int> {

    var firstTownMonsterCount = target.first
    var secondTownMonsterCount = target.second
    var killCount:Int

    //最初の街を攻撃
    when {
        firstTownMonsterCount > maxKillCount -> { //全部殺せない場合
            killCount = maxKillCount
            firstTownMonsterCount -= killCount

            return Triple(killCount.toLong(), firstTownMonsterCount, secondTownMonsterCount)
        }
        else -> {
            killCount = firstTownMonsterCount
            firstTownMonsterCount = 0
        }
    }

    //次の街を攻撃
    return when {
        secondTownMonsterCount > (maxKillCount - killCount) -> { //全部殺せない場合
            secondTownMonsterCount -= (maxKillCount - killCount)
            killCount = maxKillCount
            Triple(killCount.toLong(), firstTownMonsterCount, secondTownMonsterCount)
        }
        else ->{
            killCount += secondTownMonsterCount
            secondTownMonsterCount = 0
            Triple(killCount.toLong(), firstTownMonsterCount, secondTownMonsterCount)
        }
    }
}