Surya Siddhanta
भवन्ति शशिनो मासाः सूर्येन्दुभगणान्तरम्। रविमासोनितास् ते तु शेषाः स्युर् अधिमासकाः ॥
The number of Lunar months is equal to the difference between the revolutions of the Moon and those of the Sun; and the remainder of the Lunar Months lessened by the Solar months is the number of Adhimásas (additive months.)
english translation
bhavanti zazino mAsAH sUryendubhagaNAntaram। ravimAsonitAs te tu zeSAH syur adhimAsakAH ॥
hk transliteration by Sanscript