302 4
abdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcb
 300 3
abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabc
 10 10
fajegfaicb
 70 4
dcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbdcbd
 100 4
abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabca
 5 5
aceba
 3 4
cdb
 2 2
ab
 11 3
abcabcabcab
 3 26
yzx
 2 4
cd
 7 26
zyxzyxz
 3 3
acb
 1 2
a
 77 7
cadgbagbcaecgfaegcdbeafbacbdfgaedgcdeabgebaecbeacgfebagedcegdafdgeacegfegfegf
 1 26
o
 10 3
cbacbacbac
 10 5
abcabcabca
 26 26
ahnxdnbfcriersyzdihuecojdi
 1 2
b
 13 7
gfegfegfegfeg
 1 1
a
 6 3
acbacb
 17 26
bazyxzyxzyxzyxzyx
 12 10
abcabcabcabc
 333 5
edcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedcedc
 1 26
z
 3 3
cab
 15 11
cgjkbadjfbdaikj
 6 3
abcabc
 17 4
dabcadcbdcadbcdbc
 30 7
cedcfedcfgcfgcbadcadgfaegfacgf
 30 26
abcabcabczyxzyxzyxzyxzyxzyxzyx
 2 2
ba

acbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbac
 acbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacbacb
 fajegfaicd
 NO
 abcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcabcd
 acebc
 dab
 ba
 acbacbacbac
 zab
 da
 NO
 bac
 b
 cadgbagbcaecgfaegcdbeafbacbdfgaedgcdeabgebaecbeacgfebagedcegdafdgeacfabcabcab
 p
 NO
 abcabcabcd
 ahnxdnbfcriersyzdihuecojdk
 NO
 NO
 NO
 bacbac
 bcabcabcabcabcabc
 abcabcabcabd
 NO
 NO
 cba
 cgjkbadjfbdajba
 acbacb
 dabcadcbdcadcabca
 cedcfedcfgcfgcbadcadgfaegfadba
 abcabcabdabcabcabcabcabcabcabc
 NO

import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.math.BigInteger;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        TaskC solver = new TaskC();
        solver.solve(1, in, out);
        out.close();
    }
}

class TaskC {
    int p, n;
    OutputWriter out;

    public void solve(int testNumber, InputReader in, OutputWriter out) {
        n = in.readInt();
        p = in.readInt();
        this.out = out;
        String s = in.readString();
        for (int i = s.length() - 1; i >= 0; i--) {
            if (canTransform(s.toCharArray(), i)) {
                return;
            }
        }
        out.printLine("NO");
    }

    boolean canTransform(char[] a, int idx) {
        //out.printLine("At " + idx);
        //out.printLine(a[idx]);
        if (a[idx] - 'a' == p - 1) {
            return false;
        }
        boolean can = false;
        for (int j = a[idx] + 1; j < 'a' + p; j++) {
            can =  false;
            if (idx - 1 >= 0 && a[idx - 1] != j) {
                can = true;
                if (idx - 2 >= 0 && a[idx - 2] == j) {
                    can = false;
                }
                if (can) {
                    a[idx] = (char) j;
                    break;
                }
            }
        }
        if (!can) {
            return false;
        }
        for (int i = idx + 1; i < n; i++) {
            boolean transform = false;
            for (int j = 'a'; j < 'a' + p; j++) {
                if (i - 1 >= 0 && a[i - 1] != j) {
                    boolean ok = true;
                    if (i - 2 >= 0 && a[i - 2] == j) {
                        ok = false;
                    }
                    if (ok) {
                        a[i] = (char) j;
                        transform = true;
                        break;
                    }
                }
            }
            if (!transform)
                return false;
        }
        out.printLine(String.valueOf(a));
        return true;
    }
}

class OutputWriter {
    private final PrintWriter writer;

    public OutputWriter(OutputStream outputStream) {
        writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
    }

    public OutputWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }

    public void print(Object...objects) {
        for (int i = 0; i < objects.length; i++) {
            if (i != 0)
                writer.print(' ');
            writer.print(objects[i]);
        }
    }

    public void printLine(Object...objects) {
        print(objects);
        writer.println();
    }

    public void close() {
        writer.close();
    }

}

class InputReader {

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int read() {
        if (numChars == -1)
            throw new InputMismatchException();
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0)
                return -1;
        }
        return buf[curChar++];
    }

    public int readInt() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9')
                throw new InputMismatchException();
            res *= 10;
            res += c - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public String readString() {
        int c = read();
        while (isSpaceChar(c))
            c = read();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public boolean isSpaceChar(int c) {
        if (filter != null)
            return filter.isSpaceChar(c);
        return isWhitespace(c);
    }

    public static boolean isWhitespace(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    public interface SpaceCharFilter {
        public boolean isSpaceChar(int ch);
    }
}

