#JavaScript (ES6), <s> 83 </s> 75 bytes 0-indexed. <!-- language-all: lang-javascript --> f=(n,i,a=[p=1])=>a[n]||f(n,-~i,[...a,p+=(h=k=>k<i&&a[k]*a[i-++k]+h(k))(2)]) [Try it online!](https://tio.run/##FY1BCoMwEADvfcUeRLJNDK1XXT8SAi5WaxpJREsvar@e2tPAMDAv/vDaLW5@FyE@@pQGEkE5xWRmulukhk2w@z6ctvg6ZbTWrGZJYiRPja9dnrPx9srGFVJ6K0fhEUWJFtMQFxGA4FZBgBrKP6VE2C4AXQxrnHo9xadoWWRbOPBMs@084dFidTnSDw "JavaScript (Node.js) – Try It Online")