some sec fixes

FibonacciWindow.xaml.cs

@@ -35,7 +35,7 @@ namespace Lab
             long a = 1;
             long b = 1;
             List<long> list = new List<long>();
-            while (a < n) { list.Add(a); long next = a + b; a = b; b = next; }
+            while (a < n) { list.Add(a); if (long.MaxValue - a < b) break; long next = a + b; a = b; b = next; }
 
             OutBox.Text = string.Join(", ", list);
             InfoText.Text = $"Найдено чисел: {list.Count}.";