diff --git a/src/bin/25.rs b/src/bin/25.rs
new file mode 100644
index 0000000..2f6b926
--- /dev/null
+++ b/src/bin/25.rs
@@ -0,0 +1,62 @@
+use aoc::*;
+use itertools::{zip_eq, Itertools};
+use std::collections::BTreeMap;
+
+const INPUT: &str = include_str!("../../input/25");
+
+fn main() {
+    assert_example!(part1, "25-test", 3);
+    println!("Part 1: {}", part1(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    let (locks, keys) = parse(input);
+    locks
+        .into_iter()
+        .cartesian_product(keys)
+        .filter(|(l, k)| no_overlap(l, k))
+        .count()
+}
+
+fn no_overlap(lock: &Heights, key: &Heights) -> bool {
+    zip_eq(lock, key).all(|(&l, &k)| l + k <= 5)
+}
+
+fn parse(input: &str) -> (Vec<Heights>, Vec<Heights>) {
+    let locks = input
+        .split("\n\n")
+        .filter(|lines| is_lock(lines))
+        .map(parse_heights)
+        .collect();
+    let keys = input
+        .split("\n\n")
+        .filter(|lines| !is_lock(lines))
+        .map(parse_heights)
+        .collect();
+    (locks, keys)
+}
+
+fn is_lock(lines: &str) -> bool {
+    lines.starts_with("#####")
+}
+
+type Heights = Vec<i64>;
+
+fn parse_heights(lines: &str) -> Heights {
+    assert_eq!(lines.lines().count(), 7);
+
+    let mut height = BTreeMap::new();
+
+    for line in lines.lines() {
+        let indices = line
+            .chars()
+            .enumerate()
+            .filter(|(_i, c)| *c == '#')
+            .map(|(i, _c)| i);
+        for index in indices {
+            *height.entry(index).or_insert(-1) += 1;
+        }
+    }
+
+    height.into_values().collect()
+}