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.*; 
import java.lang.reflect.Array; 
import java.math.BigDecimal;
import java.math.BigInteger; 
import java.sql.Time;
import java.util.*; 
import java.util.Map.Entry;

import static java.lang.Math.*; 
   
public class solver implements Runnable { 
        final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null; 
        BufferedReader in; 
        PrintWriter out; 
        StringTokenizer tok = new StringTokenizer(""); 
        
        boolean HasTokens(){
            while(!tok.hasMoreTokens()){ 
                try{ 
                        tok = new StringTokenizer(in.readLine()); 
                }catch (Exception e){ 
                        return false; 
                } 
            }
            return true;
        }
          
        String readString() throws IOException{                 
            return HasTokens() ? tok.nextToken():null; 
        } 
        String readString(String delim) throws IOException{ 
            return HasTokens() ? tok.nextToken(delim):null;
        } 
         
        int readInt() throws IOException{ 
            return Integer.parseInt(readString()); 
        }   
        long readLong() throws IOException{ 
            return Long.parseLong(readString()); 
        }       
        double readDouble() throws IOException{ 
            return Double.parseDouble(readString()); 
        }  
        char readChar() throws IOException { 
            char val = 0; 
            do{ 
                val = (char) in.read(); 
            }while(val == '\n' || val == '\r'); 
            return val; 
        } 
        char readChar(String skipChars)throws IOException{ 
            char val = 0; 
            boolean ret; 
            do{ 
                val = (char) in.read(); 
                ret = false; 
                for (int i = skipChars.length()-1; i >= 0; i--){ 
                    if (skipChars.charAt(i) == val){ 
                        ret = true; 
                        break; 
                    } 
                } 
            }while(ret); 
            return val; 
        }
//----------------------------------
        int gcd(int x, int y){
            while (y != 0) {
                int temp = x%y; x = y; y = temp;
            }
            return x;
        }
//----------------------------------        
          
        public class Pair<T1,T2> { 
            public T1 x1; 
            public T2 x2; 
            public Pair(){}; 
            public Pair(T1 x1, T2 x2){ 
                this.x1 = x1; 
                this.x2 = x2; 
            } 
            public String toString(){ 
                return x1 + ":" + x2; 
            } 
        } 
        public class Point implements Comparable<Point>{ 
            public int x2; int x1; 
            public Point(){} 
            public Point(int a, int b){x1 = a;x2=b;} 
            public int compareTo(Point o) {
                return (int)-(x1*x2 - o.x1*o.x2); 
                //if (x1 == o.x1) return -(x2 -o.x2);
                //return (x1-o.x1);  
                // (x1-o.x1) - increase; -(x1-o.x1) - decrease 
            } 
            public String toString(){ 
                return x1 + ":" + x2; 
            } 
            @Override
            public int hashCode() {
                final int prime = 31;
                int result = 1;
                result = prime * result + getOuterType().hashCode();
                result = prime * result + x1;
                result = prime * result + x2;
                return result;
            }
            @Override
            public boolean equals(Object obj) {
                if (this == obj)
                    return true;
                if (obj == null)
                    return false;
                if (getClass() != obj.getClass())
                    return false;
                Point other = (Point) obj;
                if (!getOuterType().equals(other.getOuterType()))
                    return false;
                if (x1 != other.x1)
                    return false;
                if (x2 != other.x2)
                    return false;
                return true;
            }
            private solver getOuterType() {
                return solver.this;
            }
        } 
        public class Three implements Comparable<Three>{ 
            public int x1, x2, x3; 
            public Three(){} 
            public Three(int a, int b, int c){x1 = a;x2=b;x3=c;} 
            public int compareTo(Three o) {
                if(x1 == o.x1 && x2 == o.x2) return x3-o.x3;
                if(x1 == o.x1) return x2-o.x2;
                return (x1-o.x1);  
                // (x1-o.x1) - increase; -(x1-o.x1) - decrease 
            } 
            public String toString(){ 
                return x1 + ":" + x2 + ":" + x3; 
            } 
        } 
//----------------------------------------     
        int n, p;
        char[] s;
        char[] tr;
        
        public void solve() throws IOException {        
            
            n = readInt();          
            p = readInt();
            
            s = readString().toCharArray();
            tr = s.clone();
            boolean y = false;
            
            
            for (int i = n-1; i>= 0 && !y; i--){
                for (int j = s[i] - 'a' + 1; j < p; j++){
                    s[i] = (char)(j+'a');
                    if (Can(i) && Set(i+1)){
                        y = true;
                        break;
                    }
                }
            }
            
            
            if (!y){
                out.print("NO");
            }
            else{
                for (char e : s){
                    out.print(e);
                }
            }
        }
        
        boolean Set(int x){
            if (x == n) return true;
            
            for (int i = 0; i < p; i++){
                s[x] = (char)(i + 'a');
                if (Can(x) && Set(x+1)){
                    return true;
                }
            }
            return false;
        }
        
        
        boolean Can(int x){
            if (x-1 >= 0 && s[x-1] == s[x]){
                return false;
            }
            if (x-2 >= 0 && s[x-2] == s[x]){
                return false;
            }
//          if (x+1 < n && s[x+1] == s[x]){
//              return false;
//          }
//          if (x+2 < n && s[x+2] == s[x]){
//              return false;
//          }
            return !false;
        }
        
//---------------------------------------- 
        public static void main(String[] args){ 
                new Thread(null, new solver(), "", 256 * (1L << 20)).start(); 
        } 
         
        public void run(){ 
                try{ 
                        timeBegin = System.currentTimeMillis(); 
                        try{ 
                            if (ONLINE_JUDGE){ 
                                in = new BufferedReader(new InputStreamReader(System.in)); 
                                out = new PrintWriter(System.out); 
                            }  
                            else { 
                                in = new BufferedReader(new FileReader("input.txt")); 
                                out = new PrintWriter("output.txt"); 
                            } 
                        }catch(Throwable e){
                            in = new BufferedReader(new InputStreamReader(System.in)); 
                            out = new PrintWriter(System.out); 
                        }
                          
                        solve(); 
                        out.close(); 
                          
                        timeEnd = System.currentTimeMillis(); 
                        System.err.println("Time = " + (timeEnd - timeBegin)); 
                }catch (Exception e){ 
                        e.printStackTrace(System.err); 
                        System.exit(-1); 
                } 
        } 
         
        long timeBegin, timeEnd; 
   
          
//----------        
        void update(int[] doo, int v, int i, int val, int ll, int rr){ 
            if (ll == rr){ 
                doo[v] = val; 
            }else{ 
                int med = ll + ((rr-ll)>>1); 
                  
                if (i <= med){ 
                    update(doo, v<<1, i, val, ll, med); 
                }else{ 
                    update(doo, (v<<1)+1, i, val, med+1, rr); 
                } 
                doo[v] = min(doo[v<<1], doo[(v<<1)+1]); 
            } 
        } 
        long get(long[] doo, int v, int l, int r, int ll, int rr){ 
            if (l > r){ 
                return Integer.MAX_VALUE; 
            } 
            if (ll == l && r == rr){ 
                return doo[v]; 
            } 
            int med = ll + ((rr-ll)>>1);
            return min(get(doo, v<<1, l, min(r, med), ll, med),  
                    get(doo, (v<<1)+1, max(l, med+1), r, med+1, rr)); 
        }        
        
 //----------------------------------------

        int BinSearch(int[] a, int x, boolean increase){

            int first = 0;
            int last = a.length;
            
            if (increase){
                while (first <= last) {
                    int mid = first + ((last - first) >> 1);                
                    if (x <= a[mid])
                        last = mid;
                    else
                        first = mid + 1;
                }
                return last;
            }else{
                while (first <= last) {
                    int mid = first + ((last - first) >> 1);                
                    if (x > a[mid])
                        last = mid;
                    else
                        first = mid + 1;
                }
                return first;
            }
        }
        
        
//-----------------------------------------
}