diff --git a/PBDS/Codeforces PBDS example.cpp b/PBDS/Codeforces PBDS example.cpp
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+++ b/PBDS/Codeforces PBDS example.cpp	
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+/* 
+ PROBLEM   STATEMENT: 
+
+You are given an array a of n integers. Find the number of pairs (i,j) (1≤i<j≤n) where the sum of ai+aj is greater than or equal to l and less than or equal to r (that is, l≤ai+aj≤r).
+
+For example, if n=3, a=[5,1,2], l=4 and r=7, then two pairs are suitable:
+
+i=1 and j=2 (4≤5+1≤7);
+i=1 and j=3 (4≤5+2≤7).
+*/
+
+/*NOTE:  
+
+Policy based data structures in g++
+Difficulty Level : Medium
+Last Updated : 15 Jan, 2018
+The g++ compiler also supports some data structures that are not part of the C++ standard library. Such structures are called policy-based data structures. These data structures are designed for high-performance, flexibility, semantic safety, and conformance to the corresponding containers in std.
+
+To use these structures, the following lines must be added to the code:
+
+#include <ext/pb_ds/assoc_container.hpp>
+using namespace __gnu_pbds;
+
+*/
+
+#include<bits/stdc++.h>
+using namespace std;
+ 
+//common file for PBDS(Policy based data structure, here we use orderedset)
+#include<ext/pb_ds/assoc_container.hpp>
+// including tree_order_statistics_node_update
+#include<ext/pb_ds/tree_policy.hpp>
+//namespace
+using namespace __gnu_pbds;
+#define int long long
+
+//macro definition
+#define ordered_set tree<pair<int,int>, null_type, less<pair<int,int>>, rb_tree_tag, tree_order_statistics_node_update>
+ 
+#define ld long double
+#define endl "\n"
+const int mod=1000000007;
+const int MAX=100005;
+mt19937_64 rng(std::chrono::steady_clock::now().time_since_epoch().count());
+ 
+const int bit=31;
+
+signed main(){
+    int t=1;
+    cin>>t;
+    while(t--){
+        int n,l,r;
+        cin>>n>>l>>r;
+        
+        int arr[n];
+        for(int i=0;i<n;i++)
+            cin>>arr[i];
+
+        ordered_set st;
+        
+        int ans=0;
+        int tim=0;
+        
+        for(int i=n-1;i>=0;i--){
+            //number of elements strictly less than l-arr[i]
+            int lo=st.order_of_key({l-arr[i],-1});
+            
+            //number of elements strictly less than r-arr[i]
+            int hi=st.order_of_key({r-arr[i],INT_MAX});
+            st.insert({arr[i],tim++});
+            ans+=(hi-lo);
+        }
+        cout<<ans<<endl;
+    }
+    return 0;
+}
diff --git a/PBDS/readme.txt b/PBDS/readme.txt
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+Policy based data structure