Yes you can.

Combined defines exam is an exam conducted for recruiting offices to army, air force and Navy. It is conducted by UPSC. Recruitment of the candidates are to Indian Military Academy, Officers Training Academy, Indian Naval Academy and Indian Air Force Academy. Candidates who have completed their bachelor's degree from a recognised board are eligible for applying.

It doesn't matter your subject in 12th or degree for applying the exam. Minimum qualification for CDS exam is a bachelor's degree.

Hence, you are eligible.

Thank you