diff --git a/May/Day 31/1396_Design_Underground_System.java b/May/Day 31/1396_Design_Underground_System.java
new file mode 100644
index 0000000..3e6390f
--- /dev/null
+++ b/May/Day 31/1396_Design_Underground_System.java	
@@ -0,0 +1,37 @@
+class UndergroundSystem {
+    private Map<Integer, Pair<String, Integer>> checkIns = new HashMap<>();
+    private Map<Pair<String, String>, int[]> times = new HashMap<>();
+    
+    public UndergroundSystem() {
+        
+    }
+    
+    public void checkIn(int id, String stationName, int t) {
+        checkIns.put(id, new Pair(stationName, t));
+    }
+    
+    public void checkOut(int id, String stationName, int t) {
+        var startStation = checkIns.get(id).getKey();
+        var startTime = checkIns.get(id).getValue();
+        checkIns.remove(id);
+
+        var pair = new Pair(startStation, stationName);
+        var totalTime = times.containsKey(pair) ? times.get(pair)[0] : 0;
+        var dataPoints = times.containsKey(pair) ? times.get(pair)[1] : 0;
+
+        times.put(pair, new int[]{totalTime + t - startTime, dataPoints + 1});
+    }
+    
+    public double getAverageTime(String startStation, String endStation) {
+        var pair = new Pair(startStation, endStation);
+        return (double) times.get(pair)[0] / times.get(pair)[1];
+    }
+}
+
+/**
+ * Your UndergroundSystem object will be instantiated and called as such:
+ * UndergroundSystem obj = new UndergroundSystem();
+ * obj.checkIn(id,stationName,t);
+ * obj.checkOut(id,stationName,t);
+ * double param_3 = obj.getAverageTime(startStation,endStation);
+ */
\ No newline at end of file