The amount at a given time = A
The amount you began with = B
The amount of time that has passed in months = t
The number e is an important mathematical constant, approximately equal to 2.71828, that is the base of the natural logarithm. It is the limit of (1 + 1/n)n as n becomes large, an expression that arises in the study of compound interest, and can also be calculated as the sum of the infinite series.
Without knowing the starting amount and if you are taking out the interest each month or allowing it to compound I cannot give any closer answer.