Added code for nth magic number

BitManipulation/Nth_magic_number/README.md

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+# MAGIC NUMBER
+
+A magic number is defined as a number which can be expressed as a power of 5 or sum of unique powers of 5. First few magic numbers are 5, 25, 30(5 + 25), 125, 130(125 + 5), ….

BitManipulation/Nth_magic_number/nth_magic_number.cpp

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+#include <bits/stdc++.h> 
+using namespace std; 
+
+int nthMagicNo(int n) 
+{ 
+    int pow = 1, answer = 0; 
+
+    while (n) 
+    { 
+       pow = pow*5; 
+
+       if (n & 1) 
+         answer += pow; 
+
+    } 
+    return answer; 
+} 
+
+int main() 
+{ 
+    int n;
+    cin>>n; 
+    cout << "nth magic number is " << nthMagicNo(n) << endl; 
+    return 0; 
+}