Let's suppose I have a data frame with two columns, one for months and the second one for days. Here is a simple example

```
month=c(2, 4, ,7, 8, 11, 11, 12)
day=c(21,4,6,8,15,20,30)
monthday=data.frame(month,day)

```

I want to determine a number (from 1 to 365) that corresponds to the year's day. How can I do that?