diff --git a/days/src/main/kotlin/Day09.kt b/days/src/main/kotlin/Day09.kt
index 2061335..e18e731 100644
--- a/days/src/main/kotlin/Day09.kt
+++ b/days/src/main/kotlin/Day09.kt
@@ -10,23 +10,26 @@ class Day09(@Lines val input: Input<LongArray>) {
 
     private var part1Result = 0L
 
-    fun part1() = input.value
-        .toList()
-        .asSequence()
-        .windowed(size = 26)
-        .find { !isValid(it) }
-        ?.last()
-        ?.also { part1Result = it }
+    fun part1(): Long? {
+        val longs = input.value
 
-    private fun isValid(items: List<Long>): Boolean {
-        val preamble = items.take(25)
-        val last = items.last()
+        for (windowStart in 0 until longs.size - 26) {
+            val last = longs[windowStart + 25]
+            if (!isValid(windowStart, last)) {
+                part1Result = last
+                return last
+            }
+        }
 
-        for (e in preamble) {
-            for (f in preamble) {
-                if (e + f == last && e != f) {
-                    return true
-                }
+        return null
+    }
+
+    private fun isValid(windowStart: Int, last: Long): Boolean {
+        for (i in windowStart..windowStart + 25) {
+            for (j in windowStart + 1..windowStart + 25) {
+                val f = input.value[i]
+                val s = input.value[j]
+                if (f + s == last && f != s) return true
             }
         }
         return false