How to type tilde in formulas in RMarkdown

问题:

I am trying to type equations on RMarkdown to create a PDF and I want to use a tilde, how can I go about it. I am trying this but throws back a pandoc.exe: Error producing PDF from TeX source ....

---
title: "See"
date: "24 September 2016"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

follows a Bernouli distribution 
$$y_i \~ Bernouli(p_i)$$