import java.util.*

fun main() {
    q10773()
}

fun q10773() = with(Scanner(System.`in`)) {
    val inputs = List(nextInt()) {nextInt()}
    val stack = Stack<Int>()
    for (input in inputs) {
        if(input == 0) stack.pop()
        else stack.push(input)
    }
    println(stack.sum())
}