Help with resizing hard drive after failed Boot Camp

The other day I tried to install Windows 8.1 on my iMac (Mid-2011, 21.5 inch) running OS X Yosemite 10.10.1. The process froze halfway through, resulting in me losing 100 GB (which was suppose to be allocated for Windows) on my main Hard drive.

After running diskutil list, my output is…

    /dev/disk0
    #:                       TYPE NAME                    SIZE       IDENTIFIER
    0:     FDisk_partition_scheme                        *500.1 GB   disk0
    1:                  Apple_HFS Mac HD                  399.2 GB   disk0s1

As you can see, I have FDisk_partition_scheme which is 500.1 GB and my main Apple_HFS called, Mac HD which has all my files and folders, that is 399.2 GB. I wish to make my Mac HD ~500 GB so I can recover that 100 GB that has disappeared.

I also believe that I'm missing a Recovery HD. When I boot with Command + R, I am shown a spinning globe which downloads some sort of outdated recovery (because it shows the old UI). Every time I boot my iMac, I booted into Windows side (somehow?) with the message of something similar "No disk found" etc.

Any help would be much appreciated, thank you!

(DISCLAIMER: I know this has been asked before, but what I am asking [I believe] is to be different to others.)