diff --git a/day-01/internal/one/solve.go b/day-01/internal/one/solve.go
index 645f472..7ff79ae 100644
--- a/day-01/internal/one/solve.go
+++ b/day-01/internal/one/solve.go
@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"embed"
+	"fmt"
 	"io"
 	"math"
 	"regexp"
@@ -16,8 +17,11 @@ var files embed.FS
 
 var reNums = regexp.MustCompile(`[0-9]+`)
 
-func Solve() (int, error) {
-	data, _ := files.ReadFile("testdata/input.txt")
+func Solve(filename string) (int, error) {
+	data, err := files.ReadFile(fmt.Sprintf("testdata/%s", filename))
+	if err != nil {
+		return 0, err
+	}
 
 	r := bytes.NewReader(data)
 	left, right, err := parseLines(r)
diff --git a/day-01/internal/two/solve.go b/day-01/internal/two/solve.go
index 357b0d6..751636f 100644
--- a/day-01/internal/two/solve.go
+++ b/day-01/internal/two/solve.go
@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"embed"
+	"fmt"
 	"io"
 	"regexp"
 	"slices"
@@ -16,8 +17,11 @@ var files embed.FS
 
 var reNums = regexp.MustCompile(`[0-9]+`)
 
-func Solve() (int, error) {
-	data, _ := files.ReadFile("testdata/input.txt")
+func Solve(filename string) (int, error) {
+	data, err := files.ReadFile(fmt.Sprintf("testdata/%s", filename))
+	if err != nil {
+		return 0, err
+	}
 
 	r := bytes.NewReader(data)
 	left, right, err := parseLines(r)
diff --git a/day-01/main.go b/day-01/main.go
index 691e863..53f26b4 100644
--- a/day-01/main.go
+++ b/day-01/main.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"log"
 
@@ -13,13 +14,16 @@ import (
 )
 
 func main() {
-	val, err := one.Solve()
+	filename := flag.String("f", "input.txt", "input file")
+	flag.Parse()
+
+	val, err := one.Solve(*filename)
 	if err != nil {
 		log.Fatal(err)
 	}
 	fmt.Printf("solution one: %d\n", val)
 
-	val, err = two.Solve()
+	val, err = two.Solve(*filename)
 	if err != nil {
 		log.Fatal(err)
 	}