diff --git a/src/one.rs b/src/one.rs
index ce9f2df..8eaa076 100644
--- a/src/one.rs
+++ b/src/one.rs
@@ -1,10 +1,17 @@
 use crate::{GlobalArgs, Problem};
 
 #[derive(clap::Parser)]
-pub struct Args {}
+pub struct Args {
+    #[clap(default_value = "1000")]
+    pub until: u64,
+}
 
 impl Problem for Args {
+    // We could derive it mathematically, ... or write a dump loop
     fn solve(self, _: GlobalArgs) -> color_eyre::Result<()> {
-        todo!()
+        let sum: u64 = (0..self.until).filter(|x| x % 3 == 0 || x % 5 == 0).sum();
+        println!("Sum of 0..{}: {}", self.until, sum);
+
+        Ok(())
     }
 }