diff --git a/main.cpp b/main.cpp
index 025cf73..2468084 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1,10 +1,10 @@
 #include "std_lib_inc.h"
 
-const vector<string> first_digit_strings = {
+constexpr vector<string> first_digit_strings = {
 "", "ein", "zwei", "drei", "vier", "fuenf", "sechs", "sieben", "acht", "neun"
 };
 
-const vector<string> middle_digit_strings = {
+constexpr vector<string> middle_digit_strings = {
 "", "zehn", "zwanzig", "dreissig", "vierzig", "fuenfzig", "sechzig", "siebzig", "achzig", "neunzig"
 };