Add solution to problem two

two.adb

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+--
+--  Project Euler problem #2
+--
+--  Each new term in the Fibonacci sequence is generated by adding the previous
+--  two terms. By starting with 1 and 2, the first 10 terms will be:
+--
+--  1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
+--
+--  By considering the terms in the Fibonacci sequence whose values do not
+--  exceed four million, find the sum of the even-valued terms.
+
+with Ada.Text_IO;
+
+use Ada.Text_IO;
+
+procedure Two is
+    -- Upper bound of the Fibonacci sequence to consider
+    Fib_Ceiling : constant Natural := 4_000_000;
+    Fib_Previous : Natural := 0;
+    Fib_Current : Natural := 1;
+    Result  : Natural := 0;
+
+    function Is_Even (Number : in Natural) return Boolean is
+    begin
+        if (Number mod 2) = 0 then
+            return true;
+        else
+            return false;
+        end if;
+    end Is_Even;
+
+begin
+    while Fib_Current < Fib_Ceiling loop
+        declare
+            New_Value : Natural := Fib_Current + Fib_Previous;
+        begin
+            Fib_Previous := Fib_Current;
+            Fib_Current := New_Value;
+
+            if Is_Even (Fib_Current) then
+                Result := (Result + Fib_Current);
+            end if;
+        end;
+    end loop;
+
+    Put_Line (Natural'Image (Result));
+end Two;