How to ignore a single string value in an unordered map/ map?

I have a program that takes in a text file of DNA strings, splits them into kmer substrings and counts how many times a unique substring pops up. My only issue is having it recognize a string value "N" and ignore it in the file... for example a text file like so:

3 3

ACNTG

ACTG

ACTG

would split the dna sequence into 3 kmers, hence the first integer. The issue is i want to ignore the N and move on without including the N in the unique string value. so the output would be...

ACT,CTG, TGA... and so on while ignoring the N value. here is the portion of my code that I believe that portion should be included into:

    #include <fstream>
    #include <iostream>
    #include <string>
    #include <unordered_map>

       std::string kmer = "";

     std::unordered_map<std::string, int  > dna;
       for(int i = 0; i< s.length() ; ++i){
        int z= 0;
        kmer = s.substr(z,k);
        ++z;

        if (kmer.length() != k){
                    break;
              }

//DONT UNDERSTAND WHY THIS WOULDNT WORK
       if(!dna.find("N")) !=std::string::npos)){
        dna[kmer]++;
         }

     }



          for (std::unordered_map<std::string,int>::iterator it=dna.begin(); it!=dna.end(); ++it){


            std::cout << it->first << " " << it->second << std::endl;
        }



  f.close();
  return 0;
}