# TI-BASIC, 6 bytes
Beating Casio-basic by 3 bytes :)
6 byte version thanks to PT_ from cemetech (https://www.cemetech.net/forum/profile.php?mode=viewprofile&u=10064) 

    mean({N²,N

Two other, 7 byte, versions:

    .5N(N+1 
    
    .5(N²+N