Day 7 but math

src/day7.rs

@@ -10,13 +10,16 @@ enum Operation {
 }
 
 impl Operation {
+    #[allow(clippy::pedantic)]
     fn apply(self, a: u64, b: u64) -> u64 {
         match self {
             Self::Add => a + b,
             Self::Mul => a * b,
             Self::Concentration => {
-                let val = format!("{a}{b}");
-                val.parse().unwrap()
+                if b == 0 {
+                    return a * 10;
+                }
+                (a * 10_u64.pow((b as f64).log10() as u32 + 1)) + b
             }
         }
     }