Fix subtraction overflow.

src/main.rs

@@ -76,12 +76,14 @@ fn main() {
 				arr[n2] = !arr[n2];
 			}
 
+			if x > y {
 				let n3 = ((3 * x * x) - (y * y)) as usize;
-			if x > y && n3 <= (opts.num as usize) && n3 % 12 == 11 {
+				if n3 <= (opts.num as usize) && n3 % 12 == 11 {
 					arr[n3] = !arr[n3];
 				}
 			}
 		}
+	}
 
 	for i in 5..=(f64::sqrt(opts.num as f64) as u64 + 1) as usize {
 		if !arr[i as usize] {