diff --git a/src/main/java/sig/oss/.Grader.java.swp b/src/main/java/sig/oss/.Grader.java.swp
new file mode 100644
index 0000000..e736c1e
Binary files /dev/null and b/src/main/java/sig/oss/.Grader.java.swp differ
diff --git a/src/main/java/sig/oss/Grader.java b/src/main/java/sig/oss/Grader.java
index 0a29daf..12b049b 100644
--- a/src/main/java/sig/oss/Grader.java
+++ b/src/main/java/sig/oss/Grader.java
@@ -1,15 +1,24 @@
 package sig.oss;
 //Initial Grader Project
 class Grader { 
+	final static int MAX_SCORE = 50;
 	static public Grade calculate_grade(int mid, int fin) {
 		int tot;
 		Grade grade=Grade.F;
 		tot = mid+fin;
-		if (tot>=80) grade=Grade.A;
-		else if (tot>=70)grade=Grade.B;
-		else if (tot>=60)grade=Grade.C;
-		else grade=Grade.F;
+		if(check_score(mid) && check_score(fin)){
+			tot = mid+fin;
+			if (tot>=80) grade=Grade.A;
+			else if (tot>=70)grade=Grade.B;
+			else if (tot>=60)grade=Grade.C;
+			else grade=Grade.F;
+		}
 		return grade;
 	}
+
+	private static boolean check_score(int score){
+		if(score>=0 && score <=MAX_SCORE) return true;
+		else throw new InvalidInputException();
+	}
 }
 
diff --git a/src/main/java/sig/oss/InvalidInputException.java b/src/main/java/sig/oss/InvalidInputException.java
new file mode 100644
index 0000000..ba0559f
--- /dev/null
+++ b/src/main/java/sig/oss/InvalidInputException.java
@@ -0,0 +1,5 @@
+package sig.oss;
+
+class InvalidInputException extends RuntimeException{ 
+}
+