dsp core - FFT of a $N$-length real sequence via FFT of a $N/2$-length complex sequence

I am doing FFT. The one way I did is to put real part in one array and the imaginary part as zeros and then calculated the FFT. But this take up lots of resources. Now I want to calculate FFT by Dividing Real Input by 2 in the form of ODD and EVEN samples and put even in real array and odd in Imaginary array. Then calculate the FFT. But after the FFT calculation how can I combine them to get the same results as I got when I used imaginary as zero's.

Any idea??