import java.io.*;
class decode
{
public static void main (String args[])throws IOException
{
int b,c,p=0,m=0,l,i,j,n,d=0,r=0;
String s;
InputStreamReader x=new InputStreamReader(System.in);
BufferedReader y= new BufferedReader (x);
System.out.println("ENTER THE LENGTH OF THE CODE");
n=Integer.parseInt(y.readLine());
if(n>6)
{
p=-1;
System.out.println("INVALID_length should not exceed 6 characters");
}
if(n<=6)
{
System.out.println("ENTER THE CODE");
s=y.readLine();
l=s.length();
if(n!=l)
{
d=-1;
System.out.println("INVALID_length not same as specified");
}
for(i=0;i<l-1;i++)
{
for(j=i+1;j<l;j++)
{
b=(int)s.charAt(i);
c=(int)s.charAt(j);
if(b>=97 && b<=122)
{
p=-1;
}
if(p==-1 && d==0)
{
System.out.println("INVALID_only uppercase letters permitted");
d=-1;
}
if(b==c)
{
r=-1;
if(r==-1 && d==0)
{
System.out.println("INVALID_repetition of characters not allowed");
d=-1;
}
}
}
}
if(d!=-1)
{
for(i=0;i<l-1;i++)
{
b=(int)s.charAt(i);
c=(int)s.charAt(i+1);
if(c!=(b+2))
{
m=-1;
}
}
if(m==0)
{
System.out.println("VALID");
}
else
{
System.out.println("INVALID_only alternate letters valid");
}
}
}
}