Software is installed, but "which" returns "not found"

Question

I'm trying to work with the statistical package R. R is already installed. sudo apt-get install r-base-dev returns:
"r-base-dev is already the newest version (3.4.3-1xenial0). 0 upgraded, 0 newly installed, 0 to remove and 23 not upgraded."
However, which R gives me "R not found".

Now, how can I locate R? Is it really installed?

(Ubuntu 16.04)