diff --git a/20201231/main.go b/20201231/main.go
new file mode 100644
index 0000000..99f877c
--- /dev/null
+++ b/20201231/main.go
@@ -0,0 +1,44 @@
+package main
+
+import (
+	"strconv"
+	"strings"
+)
+
+func main() {
+
+}
+
+func process(n int) string {
+	ret := ""
+	for i := 1; i <= n; i++ {
+		ret = say(ret)
+	}
+	return ret
+}
+
+func say(num string) string {
+	if num == "" {
+		return "1"
+	}
+	var nums []string
+	var n []uint8
+	last := num[0]
+	for i := 0; i < len(num); i++ {
+		if last == num[i] {
+			n = append(n, num[i])
+		} else {
+			nums = append(nums, string(n))
+			last = num[i]
+			n = nil
+			n = append(n, num[i])
+		}
+	}
+	nums = append(nums, string(n))
+	ret := strings.Builder{}
+	for _, s := range nums {
+		ret.WriteString(strconv.Itoa(len(s)))
+		ret.WriteByte(s[0])
+	}
+	return ret.String()
+}
diff --git a/20201231/main_test.go b/20201231/main_test.go
new file mode 100644
index 0000000..5b07949
--- /dev/null
+++ b/20201231/main_test.go
@@ -0,0 +1,97 @@
+package main
+
+import "testing"
+
+func Test_say(t *testing.T) {
+	type args struct {
+		num string
+	}
+	tests := []struct {
+		name string
+		args args
+		want string
+	}{
+		{
+			"test1",
+			args{
+				num: "",
+			},
+			"1",
+		},
+		{
+			"test2",
+			args{
+				num: "1",
+			},
+			"11",
+		},
+		{
+			"test3",
+			args{
+				num: "3322251",
+			},
+			"23321511",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := say(tt.args.num); got != tt.want {
+				t.Errorf("say() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_process(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want string
+	}{
+		{
+			"test1",
+			args{
+				1,
+			},
+			"1",
+		},
+		{
+			"test2",
+			args{
+				2,
+			},
+			"11",
+		},
+		{
+			"test3",
+			args{
+				3,
+			},
+			"21",
+		},
+		{
+			"test4",
+			args{
+				4,
+			},
+			"1211",
+		},
+		{
+			"test4",
+			args{
+				5,
+			},
+			"111221",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := process(tt.args.n); got != tt.want {
+				t.Errorf("process() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}