diff --git a/in/9.txt b/in/9.txt
new file mode 100644
index 0000000..0ad2fcb
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diff --git a/source/app.d b/source/app.d
index 87c03dc..0222835 100644
--- a/source/app.d
+++ b/source/app.d
@@ -15,6 +15,7 @@ import day5;
 import day6;
 import day7;
 import day8;
+import day9;
 import dayutil;
 
 immutable string progName = "aoc-2020";
@@ -28,6 +29,7 @@ Variant function(int, File, bool, string[])[] programs = [
 	&day6.run,
 	&day7.run,
 	&day8.run,
+	&day9.run,
 ];
 
 void printUsage(string name) {
@@ -71,14 +73,20 @@ void main(string[] args) {
 	}
 
 	File file;
-	if (args.length < 4 || args[3] == "-") {
-		file = stdin;
-	} else {
-		try {
+	try {
+		if (args.length < 4)  {
+			if (bigboy) {
+				file = File("in/bigboy/%d.txt".format(day), "rb");
+			} else {
+				file = File("in/%d.txt".format(day), "rb");
+			}
+		} else if(args[3] == "-") {
+			file = stdin;
+		} else {
 			file = File(args[3], "rb");
-		} catch (ErrnoException e) {
-			printUsage(args[0], "Error %d while opening input file: %s".format(e.errno, e.message));
 		}
+	} catch (ErrnoException e) {
+		printUsage(args[0], "Error %d while opening input file: %s".format(e.errno, e.message));
 	}
 	
 	try {
diff --git a/source/day9.d b/source/day9.d
new file mode 100644
index 0000000..f39de5f
--- /dev/null
+++ b/source/day9.d
@@ -0,0 +1,31 @@
+import std.array;
+import std.algorithm;
+import std.range;
+import std.stdio;
+import std.traits;
+import std.variant;
+
+import dayutil;
+
+Variant run(int part, File input, bool bigboy, string[] args) {
+	import std.conv;
+	auto numbers = input.byLineCopy.array.map!(to!uint);
+	return Variant(parts!uint(part,
+				() => part1(numbers)));
+}
+
+T part1(Range, T = ElementType!Range)(Range range, int preambleSize = 25) 
+	if (isForwardRange!Range && isNumeric!T) {
+	foreach(window; range.slide!(No.withPartial)(preambleSize + 1)) {
+		if(!window[0..preambleSize].any!(e => window.canFind!(x => e + x == window[preambleSize]))) {
+			return window[preambleSize];
+		}
+	}
+	return -1;
+}
+
+unittest {
+	int[] numbers = [35, 20, 15, 25, 47, 40, 62, 55, 65, 95, 102, 117, 150, 182, 127, 219, 299, 
+		277, 309, 576];
+	assert(part1(numbers, 5) == 127);
+}