diff --git a/2020/src/day15.rs b/2020/src/day15.rs
index a90e653..a8abd0f 100644
--- a/2020/src/day15.rs
+++ b/2020/src/day15.rs
@@ -3,22 +3,22 @@ use std::io::Read;
 use crate::common::numbers_and_stuff;
 use crate::Solution;
 
-fn nth_number(start: &[usize], n: usize) -> usize {
+fn nth_number(start: &[u32], n: u32) -> u32 {
     // Don't want to special case this
-    assert!(n >= start.len());
+    assert!(n as usize >= start.len());
 
-    let mut history = vec![0; n];
+    let mut history = vec![0; n as usize];
 
     let mut prev = 0;
 
     for (i, &n) in start.iter().enumerate() {
-        history[prev] = i;
+        history[prev as usize] = i as u32;
         prev = n;
     }
 
-    for i in start.len()..n {
-        let last_seen = history[prev];
-        history[prev] = i;
+    for i in (start.len() as u32)..n {
+        let last_seen = history[prev as usize];
+        history[prev as usize] = i;
 
         prev = if last_seen == 0 { 0 } else { i - last_seen };
     }
@@ -31,13 +31,13 @@ pub struct Day15;
 
 impl Solution for Day15 {
     fn part1(&mut self, input: &mut dyn Read) -> String {
-        let numbers: Vec<usize> = numbers_and_stuff(input);
+        let numbers: Vec<u32> = numbers_and_stuff(input);
 
         nth_number(&numbers, 2020).to_string()
     }
 
     fn part2(&mut self, input: &mut dyn Read) -> String {
-        let numbers: Vec<usize> = numbers_and_stuff(input);
+        let numbers: Vec<u32> = numbers_and_stuff(input);
 
         nth_number(&numbers, 30000000).to_string()
     }