[R] counting the number of rows that satisfy a certain criteria

[Kate Ignatius]

I have 4 columns, and about 300K plus rows with 0s and 1s.

I'm trying to count how many rows satisfy a certain criteria... for
instance, how many rows are there that have the first column == 1 as
well as the second column == 1.

I've tried using rowSums and colSums but it keeps giving me this type of error:

Error in rowSums(X[1] == 1 & X[2] == 1) :
  'x' must be an array of at least two dimensions

Thanks in advance!