diff --git a/Arrays/P05_CheckForPairSum.py b/Arrays/P05_CheckForPairSum.py
index 45b42e5..d71e6db 100644
--- a/Arrays/P05_CheckForPairSum.py
+++ b/Arrays/P05_CheckForPairSum.py
@@ -4,8 +4,10 @@
 # in S whose sum is exactly x.
 
 def checkSum(array, sum):
-    # sort the array in descending order
-    array = quickSort(array)
+    # sort the array in ascending order
+    # new changes : made use of Python's inbuilt Merge Sort method
+    # Reason for such change : Worst case Time complexity of Quick Sort is O(n^2) whereas Worst Case Complexity of Merge Sort is O(nlog(n))
+    array = sorted(array)
 
     leftIndex = 0
     rightIndex = len(array) - 1
@@ -20,14 +22,14 @@ def checkSum(array, sum):
 
     return False, False
 
-def quickSort(array):
-    if len(array) <= 1:
-        return array
-    pivot = array[len(array) // 2]
-    left = [x for x in array if x < pivot]
-    middle = [x for x in array if x == pivot]
-    right = [x for x in array if x > pivot]
-    return quickSort(left) + middle + quickSort(right)
+##def quickSort(array):
+##    if len(array) <= 1:
+##        return array
+##    pivot = array[len(array) // 2]
+##    left = [x for x in array if x < pivot]
+##    middle = [x for x in array if x == pivot]
+##    right = [x for x in array if x > pivot]
+##    return quickSort(left) + middle + quickSort(right)
 
 if __name__ == '__main__':
     myArray = [10, 20, 30, 40, 50]
@@ -37,4 +39,4 @@ def quickSort(array):
     if(number1 and number2):
         print('Array has elements:', number1, 'and', number2, 'with sum:', sum)
     else:
-        print('Array doesn\'t has elements with the sum:', sum)
+        print('Array doesn\'t have elements with the sum:', sum)
diff --git a/Dynamic Programming/P04_SieveOfEratosthenes.py b/Dynamic Programming/P04_SieveOfEratosthenes.py
new file mode 100644
index 0000000..4c980fd
--- /dev/null
+++ b/Dynamic Programming/P04_SieveOfEratosthenes.py	
@@ -0,0 +1,32 @@
+
+# Python program to print all primes smaller than or equal to 
+# n using Sieve of Eratosthenes 
+  
+def sieve_of_eratosthenes(n): 
+      
+    # Create a boolean array "prime[0..n]" and initialize 
+    #  all entries it as true. A value in prime[i] will 
+    # finally be false if i is Not a prime, else true. 
+    prime = [True for i in range(n+1)] 
+    p = 2
+    while (p * p <= n): 
+          
+        # If prime[p] is not changed, then it is a prime 
+        if (prime[p] == True): 
+              
+            # Update all multiples of p 
+            for i in range(p * 2, n+1, p): 
+                prime[i] = False
+        p += 1
+      
+    # Print all prime numbers 
+    for p in range(2, n): 
+        if prime[p]: 
+            print (p), 
+  
+# driver program 
+if __name__=='__main__': 
+    n = 30
+    print ("Following are the prime numbers smaller"), 
+    print ("than or equal to", n )
+    sieve_of_eratosthenes(n)