


const fs = require("fs");
const filePath = process.platform === "linux" ? "/dev/stdin" : "./예제.txt";
let input = fs.readFileSync(filePath).toString().trim().split('\n');
// const fs = require('fs');
// let input = fs.readFileSync('/dev/stdin').toString().trim().split('\n');
input = input.map((el) => +el)
const K = input.shift();
let stack = [];
for (let i = 0; i < K; i++) {
if (input[i] !== 0) {
stack.push(input[i])
}
else if (input[i] === 0) {
stack.pop();
}
}
if (stack.length > 0) {
console.log(stack.reduce((a, b) => a + b))
} else {
console.log(0)
}