diff --git a/cpp/phone-number/phone_number.cpp b/cpp/phone-number/phone_number.cpp
index 35785b3..73c581c 100644
--- a/cpp/phone-number/phone_number.cpp
+++ b/cpp/phone-number/phone_number.cpp
@@ -5,9 +5,9 @@
 
 using namespace std;
 
-static const int number_size     = 10;
-static const int area_code_size  = 3;
-static const int first_part_size = 3;
+static const size_t number_size     = 10;
+static const size_t area_code_size  = 3;
+static const size_t first_part_size = 3;
 
 static const char invalid_number[] = "0000000000";