This is the 2962nd most frequent Russian word.
гривны
"Grivny" (гривны) refers to Ukrainian currency, the hryvnia.
The word 'гривны' in this context is in the genitive singular form (гривен) used to indicate the cost (plural count).
Я потратил десять гривен на книги.
I spent ten hryvnias on books.
Here, 'гривны' is in the nominative plural, used when referring to a subject that is plural.
Это ваши гривны, которые вы оставили на столе.
These are your hryvnias that you left on the table.
In this case, 'гривны' appears in genitive singular (гривен) after the numeral indicating the amount of currency.