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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.BitSet;

/**
 * Created by michal on 17.11.16.
 */
public class slo {

    private static int numbersOfbits(long x) {
        return Long.SIZE - Long.numberOfLeadingZeros(x);
    }

    private static int compareNumbers(BitSet a, BitSet b) {
        /*
        -1 if a < b
        0 if a == b
        +1 if a > b
         */
        int num_bits_a = a.length();
        int num_bits_b = b.length();
        if (num_bits_a < num_bits_b) {
            return -1;
        } else if(num_bits_a > num_bits_b) {
            return 1;
        } else {
            for (int i=num_bits_a; i >= 0; --i) {
                if(a.get(i) && !b.get(i)) {
                    return 1;
                }
                if(!a.get(i) && b.get(i)) {
                    return -1;
                }
            }
        }
        return 0;
    }

    private static BitSet add(BitSet a, BitSet b, boolean isBneg) {
        int a_len = a.length();
        int b_len = b.length();

        BitSet x;
        BitSet y;

        if (a_len < b_len) {
            x = b;
            y = a;
        } else {
            x = a;
            y = b;
        }

        BitSet result = new BitSet(a_len + 1);
        boolean over = false;

        for (int i=0,j=0; i<a_len; ++i, ++j) {
            if (x.get(i) != y.get(i)) {
                if (!over) {
                    // 1 + 0 = 1
                    result.set(i);
                }
            } else if (x.get(i) && y.get(i)) {
                if (over) {
                    // 1 + 1 +1 = 1
                    result.set(i);
                } else {
                    // 1 + 1 = 0
                    over = true;
                }
            } else {
                if (over) {
                    //1 + 0 + 0 = 1
                    result.set(i);
                    over = false;
                }
            }
        }

        if (over && !isBneg) {
            result.set(a_len);
        }

        return result;
    }

    private static BitSet numToBitSet(long x) {
        int num_bits = numbersOfbits(x);
        BitSet bitSet = new BitSet(num_bits);
        for (int i = 0; i < num_bits; ++i) {
            if (x % 2 != 0) {
                bitSet.set(i);
            }
            x >>= 1;
        }
        return bitSet;
    }

    private static int calcLetter(int prev, boolean lower) {
        //lower == true  => L
        //lower == false => R
        int c = 0;
        if (prev == 0) { //a
            if(lower) {
                c = 1;  //b
            } else {
                c = 2;  //c
            }
        }
        if (prev == 1) { //b
            if(lower) {
                c = 0;  //a
            } else {
                c = 2;  //c
            }
        }
        if (prev == 2) { //c
            if(lower) {
                c = 0;  //a
            } else {
                c = 1;  //b
            }
        }
        return c;
    }

    public static void main(String[] args) throws IOException {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        String[] values = input.readLine().split(" ");
        int n = Integer.parseInt(values[0]); //10^6
        long k = Long.parseLong(values[1]); //10^18

        BitSet s_bits = new BitSet(n);
        s_bits.set(0, n);

        BitSet k_bits = numToBitSet(k);
        int k_bits_num = k_bits.length();
        
        BitSet s_bits_neg = new BitSet(k_bits_num);
        s_bits_neg.set(0);
        s_bits_neg.set(k_bits_num - 1);

        int begin = 0;
        while (compareNumbers(k_bits, s_bits) > 0) {
            begin += 1;
            k_bits = add(k_bits, s_bits_neg, true);
        }

        if (begin > 2) {
            System.out.print("NIE");
            return;
        }
        else {
            System.out.print((char) (begin + 'a'));
        }

        while (k > 1) {
            s_bits.set(s_bits.length() - 1, false);  //7  3
            if (s_bits.length() == 1) {
                return;
            }
            int compare = compareNumbers(s_bits, k_bits);
            if (compare > 0) {
                begin = calcLetter(begin, true);   //2*
                k_bits = add(k_bits, negate(s_bits, k_bits.length()), true);
            }
            else if (compare <= 0) {
                begin = calcLetter(begin, false);  //1*
            }
            System.out.print((char) (begin + 'a'));
            --k;
        }
    }

    private static BitSet negate(BitSet s_bits, int length) {
        BitSet neg = new BitSet(length);
        boolean doNegate = false;
        for (int i=0; i<length; ++i) {
            if (s_bits.get(i)) {
                if(!doNegate) {
                    doNegate = true;
                    neg.set(i, true);
                }
            } else {
                if(doNegate) {
                    neg.set(i, true);
                }
            }
        }
        return neg;
    }

}