InverseModulus

Class name: InverseModulus
Method name: getMod
Parameters: String[]
Returns: int

Implement a class InverseModulus, which contains a method getMod. getMod takes
as parameters a String[] of the form "a,b" and returns the smallest
non-negative integer x such that x%a==b for all elements in the String[].

The String[] will contain elements of the form "int,int", like "19,3", "20,4",
and "5,1".  There is exactly one comma and no space between the integers.  The
first string "19,3" means x satisfies x % 19 == 3, likewise "20,4" means x % 20
== 4. All conditions in the String[] must be met.  Note: Quotes are included
for clarity only and are not included in the String[] elements.

If there is no number which meets the criterion specified, the method returns
-1.

The method signature is:
public int getMod(String[] al);

The String[] contains between 1 and 5 elements, inclusive.
Each String in the String[] of the correct form: "int,int".
The first number within each String is less than 20 and greater than 0.
The second number within each String is less than 20 and greater than or equal
to 0.

Examples:
-If the String[] is {"2,1","3,1","5,1"}, the method returns 1, since 1 is the
first non-negative integer x such that:
        x % 2 == 1, x % 3 == 1, and x % 5 == 1.

-If the String[] is {"7,2", "9,3"}, the method returns 30, since 30 is the
first non-negative integer x such that:
        x % 7 == 2, and x % 9 == 3.

-If the String[] is {"4,3", "8,4"}, the method returns -1, since there are no
integers x such that:
        x % 4 == 3, and x % 8 == 4.