diff --git a/cpp/phone-number/phone_number.cpp b/cpp/phone-number/phone_number.cpp
index 73c581c..2b30287 100644
--- a/cpp/phone-number/phone_number.cpp
+++ b/cpp/phone-number/phone_number.cpp
@@ -7,7 +7,7 @@ using namespace std;
 
 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 size_t exchange_size   = 3;
 
 static const char invalid_number[] = "0000000000";
 
@@ -23,8 +23,7 @@ phone_number::phone_number(const string &str)
 
         number_.erase(number_.begin());
     }
-
-    if (number_.size() != number_size) {
+    else if (number_.size() != number_size) {
 
         number_ = invalid_number;
     }
@@ -43,7 +42,7 @@ string phone_number::area_code() const
 phone_number::operator string() const
 {
     return string("(") + area_code() + ") "
-        + number_.substr(area_code_size, first_part_size)
+        + number_.substr(area_code_size, exchange_size)
         + '-'
-        + number_.substr(area_code_size + first_part_size, string::npos);
+        + number_.substr(area_code_size + exchange_size);
 }