diff --git a/src/bin/d21p1.rs b/src/bin/d21p1.rs
new file mode 100644
index 0000000..7364953
--- /dev/null
+++ b/src/bin/d21p1.rs
@@ -0,0 +1,7 @@
+use aoc22::{day21, util};
+
+pub fn main() {
+    let monkeys = day21::parse_monkeys(&util::parse_input());
+
+    println!("root yells: {}", day21::yell(&monkeys, "root"));
+}
diff --git a/src/day21.rs b/src/day21.rs
new file mode 100644
index 0000000..537823c
--- /dev/null
+++ b/src/day21.rs
@@ -0,0 +1,64 @@
+use std::collections::HashMap;
+
+use regex::Regex;
+
+#[derive(Debug)]
+pub enum Operation {
+    Add,
+    Sub,
+    Mul,
+    Div,
+}
+
+impl Operation {
+    pub fn from(s: &str) -> Operation {
+        match s {
+            "+" => Operation::Add,
+            "-" => Operation::Sub,
+            "*" => Operation::Mul,
+            "/" => Operation::Div,
+            _ => panic!("Unknown operation {}", s),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub enum Monkey {
+    Number(isize),
+    Calculate(String, String, Operation),
+}
+
+pub fn parse_monkeys(input: &String) -> HashMap<String, Monkey> {
+    let mut result = HashMap::new();
+
+    let re = Regex::new(r"^(\w+): (?:(\d+)|(\w+) ([+\-*/]) (\w+))$").unwrap();
+    for line in input.lines() {
+        let captures = re.captures(line).unwrap();
+        let name = captures.get(1).unwrap().as_str();
+        let monkey = if let Some(num) = captures.get(2) {
+            Monkey::Number(num.as_str().parse().unwrap())
+        } else {
+            Monkey::Calculate(
+                captures.get(3).unwrap().as_str().to_owned(),
+                captures.get(5).unwrap().as_str().to_owned(),
+                Operation::from(captures.get(4).unwrap().as_str()),
+            )
+        };
+        result.insert(name.to_owned(), monkey);
+    }
+
+    result
+}
+
+pub fn yell(monkeys: &HashMap<String, Monkey>, monkey: &str) -> isize {
+    let monkey = &monkeys[monkey];
+    match monkey {
+        Monkey::Number(n) => *n,
+        Monkey::Calculate(a, b, op) => match op {
+            Operation::Add => yell(monkeys, a) + yell(monkeys, b),
+            Operation::Sub => yell(monkeys, a) - yell(monkeys, b),
+            Operation::Mul => yell(monkeys, a) * yell(monkeys, b),
+            Operation::Div => yell(monkeys, a) / yell(monkeys, b),
+        },
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index e63c09b..7658860 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -11,6 +11,7 @@ pub mod day18;
 pub mod day19;
 pub mod day2;
 pub mod day20;
+pub mod day21;
 pub mod day3;
 pub mod day4;
 pub mod day5;