Given a number N, the task is to convert the given number into a Cypher string on the basis of below conditions:

- If N is a semiprime, then change every digit at even places of N to it’s corresponding matched alphabet as shown below.
- If N can be written as a sum of two primes, then change every digit at odd places of N to it’s corresponding matched alphabet as shown below.
- If both the condition satisfy the concatenate the above two strings formed.
- If N can’t satisfy the above three criteria, then print “-1”. Below is the list of matched character:

**Examples:**

Input: N = 61

Output: 6B

Explanation:

Since 61 can be expressed as a sum of two primes: 61 = 2 + 59

Therefore, the resultant string after changing the character at even index is “6B”.

Input: N = 1011243

Output: B0B1C4D

Explanation:

Since 1011243 is Semiprime number: 1011243 = 3 * 337081

Therefore, the resultant string after change the character at even index is “B0B1C4D”.

**Approach:**

- Check if the given number N is semi prime or not by using the approach discussed in this article. If yes, then do the following:
- Convert the given number N to string(say str) using to_string() function.
- Traverse the above string formed and changed the characters at even index as:

```
str[i] = char((str[i] - '0') + 65)
```

- Print the new string formed.
- Check if the given number N can be expressed as a sum of two prime numbers or not using the approach discussed in this article. If yes, then do the following:
- Convert the given number N to string(say str) using to_string() function.
- Traverse the above string formed and changed the characters at odd index as:

```
str[i] = char((str[i] - '0') + 65)
```

- Print the new string formed.
- If the above two condition doesn’t satisfy then we can’t form Cypher String. Print “-1”.

Below is the implementation of the above approach:

```
// C++ program for the above approach
#include "bits/stdc++.h"
using namespace std;
// Function to check whether a number
// is prime or not
bool isPrime(int n)
{
if (n <= 1)
return false;
for (int i = 2; i <= sqrt(n); i++) {
if (n % i == 0)
return false;
}
return true;
}
// Function to check if a prime number
// can be expressed as sum of
// two Prime Numbers
bool isPossibleSum(int N)
{
// If the N && (N-2) is Prime
if (isPrime(N)
&& isPrime(N - 2)) {
return true;
}
else {
return false;
}
}
// Function to check semiPrime
bool checkSemiprime(int num)
{
int cnt = 0;
// Loop from 2 to sqrt(num)
for (int i = 2; cnt < 2
&& i * i <= num;
++i) {
while (num % i == 0) {
num /= i,
// Increment the count of
// prime numbers
++cnt;
}
}
// If num is greater than 1, then
// add 1 to it
if (num > 1) {
++cnt;
}
// Return '1' if count is 2 else
// return '0'
return cnt == 2;
}
// Function to make the Cypher string
void makeCypherString(int N)
{
// Resultant string
string semiPrime = "";
string sumOfPrime = "";
// Make string for the number N
string str = to_string(N);
// Check for semiPrime
if (checkSemiprime(N)) {
// Traverse to make Cypher string
for (int i = 0; str[i]; i++) {
// If index is odd add the
// current character
if (i & 1) {
semiPrime += str[i];
}
// Else current character is
// changed
else {
semiPrime
+= char(
str[i] - '0' + 65);
}
}
}
// Check for sum of two primes
if (isPossibleSum(N)) {
// Traverse to make Cypher string
for (int i = 0; str[i]; i++) {
// If index is odd then
// current character is
// changed
if (i & 1) {
sumOfPrime
+= char(
str[i] - '0' + 65);
}
// Else add the current
// character
else {
sumOfPrime += str[i];
}
}
}
// If the resultant string is ""
// then print -1
if (semiPrime + sumOfPrime == "") {
cout << "-1";
}
// Else print the resultant string
else {
cout << semiPrime + sumOfPrime;
}
}
// Driver Code
int main()
{
// Given Number
int N = 1011243;
// Function Call
makeCypherString(N);
return 0;
}
```

**Output:**

```
B0B1C4D
```

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