diff --git a/src/bin/day9.rs b/src/bin/day9.rs
index 626f4f6..a7324cd 100644
--- a/src/bin/day9.rs
+++ b/src/bin/day9.rs
@@ -2,7 +2,7 @@
 #![forbid(elided_lifetimes_in_paths, unsafe_code)]
 
 use aoc23::read;
-use chumsky::{prelude::*, text::int};
+use chumsky::prelude::*;
 
 struct History(Vec<i64>);
 
@@ -31,8 +31,7 @@ fn parser() -> impl Parser<char, Vec<History>, Error = Simple<char>> {
 
 impl History {
 	fn diff(&self) -> Self {
-		// intentionally reserve one space extra for .extend()
-		let mut diffs = Vec::with_capacity(self.0.len());
+		let mut diffs = Vec::with_capacity(self.0.len() - 1);
 		if self.0.len() > 1 {
 			for i in 1 .. self.0.len() {
 				diffs.push(self.0[i] - self.0[i - 1]);
@@ -41,13 +40,23 @@ impl History {
 		Self(diffs)
 	}
 
-	fn extend(&mut self) {
+	fn extend_last(&self) -> i64 {
 		if self.0.iter().any(|num| *num != 0) {
-			let mut diff = self.diff();
-			diff.extend();
-			self.0.push(self.0.last().unwrap() + diff.0.last().unwrap());
+			let diff = self.diff();
+			let last = diff.extend_last();
+			self.0.last().unwrap() + last
 		} else {
-			self.0.push(0);
+			0
+		}
+	}
+
+	fn extend_first(&self) -> i64 {
+		if self.0.iter().any(|num| *num != 0) {
+			let diff = self.diff();
+			let first = diff.extend_first();
+			self.0.first().unwrap() - first
+		} else {
+			0
 		}
 	}
 }
@@ -55,12 +64,14 @@ impl History {
 fn main() -> anyhow::Result<()> {
 	let histories = read("inputs/day9.txt", parser())?;
 
-	let mut sum = 0;
-	for mut h in histories {
-		h.extend();
-		sum += h.0.last().unwrap();
+	let mut sum_last = 0;
+	let mut sum_first = 0;
+	for h in histories {
+		sum_last += h.extend_last();
+		sum_first += h.extend_first();
 	}
-	println!("{sum}");
+	println!("{sum_last}");
+	println!("{sum_first}");
 
 	Ok(())
 }