diff --git a/introductory.go b/introductory.go
index c6ab52d..ab0688e 100644
--- a/introductory.go
+++ b/introductory.go
@@ -19,3 +19,29 @@ func WeirdAlgo(n int) string {
 
 	return fmt.Sprint(seq)
 }
+
+func MissingNumber(n int, numbers []int) int {
+
+	if n != len(numbers)+1 {
+		return -1
+	}
+
+	present := make([]bool, n)
+
+	for _, i := range numbers {
+
+		if i > n {
+			return -2
+		}
+
+		present[i-1] = true
+	}
+
+	for i, b := range present {
+		if !b {
+			return i + 1
+		}
+	}
+
+	return -3
+}
diff --git a/introductory_test.go b/introductory_test.go
index b2c87cf..a20c4fa 100644
--- a/introductory_test.go
+++ b/introductory_test.go
@@ -9,3 +9,8 @@ import (
 func TestWeirdAlgo(t *testing.T) {
 	assert.Equal(t, "[3 10 5 16 8 4 2 1]", WeirdAlgo(3))
 }
+
+func TestMissingNumber(t *testing.T) {
+	assert.Equal(t, 4, MissingNumber(5, []int{2, 3, 1, 5}))
+	assert.Equal(t, 7, MissingNumber(9, []int{2, 3, 1, 5, 4, 6, 9, 8}))
+}