export {}
const input:number[] = require("fs").readFileSync(0,"utf8").toString().trim().split('\n').map(Number);
let stack:number[] = [];
input.forEach((v,i) => {
    if(i!==0){
        if(v===0){
            stack.pop();
        }
        else {
            stack.push(v);
        }
    }
});
console.log(stack.reduce((acc,v) => acc + v, 0));