diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7d72bf3..d343e5a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -4,7 +4,7 @@ enable_testing()
 
 add_library(common_options INTERFACE)
 target_compile_features(common_options INTERFACE
-  cxx_std_20
+  cxx_std_17
 )
 target_compile_options(common_options INTERFACE
   -Wall
diff --git a/src/bignum/bignum.cc b/src/bignum/bignum.cc
index cadc170..2121cdc 100644
--- a/src/bignum/bignum.cc
+++ b/src/bignum/bignum.cc
@@ -3,7 +3,6 @@
 #include <algorithm>
 #include <iostream>
 #include <iterator>
-#include <span>
 
 #include <cassert>
 #include <cmath>
@@ -83,17 +82,18 @@ digits_type do_addition(digits_type const& lhs, digits_type const& rhs) {
         ++it2;
     }
 
-    auto leftover = [=]() {
-        if (it1 != end1) {
-            return std::span(it1, end1);
-        }
-        return std::span(it2, end2);
-    }();
+    auto it = it1;
+    auto end = end1;
+    if (it1 == end1) {
+        it = it2;
+        end = end2;
+    }
 
-    for (auto value : leftover) {
-        int addition = value + carry;
+    while (it != end) {
+        int addition = *it + carry;
         carry = addition / BASE;
         res.push_back(addition % BASE);
+        ++it;
     }
 
     if (carry != 0) {