diff --git a/day6/p1.c b/day6/p1.c
index b5faf25..2559947 100644
--- a/day6/p1.c
+++ b/day6/p1.c
@@ -7,7 +7,7 @@
 
 #define ISDIGIT(c) (c >= 0x30 && c <= 0x39)
 
-int64_t parse_number(char *end);
+inline int64_t parse_number(const char *end);
 
 
 int main(int argc, char **argv) {
@@ -96,7 +96,7 @@ int main(int argc, char **argv) {
     return 0;
 }
 
-int64_t parse_number(char *end) {
+inline int64_t parse_number(const char *end) {
     int64_t num = 0;
 
     for (size_t m = 1;; m *= 10) {