#include<iostream>
using namespace std;
int main()
{
    int i=0;
    char arr[10];
    cin>>arr;
    while(arr[i]!='\0')
    {
        if(arr[i]<=109&&arr[i]>=97){
            i++;
        }
        else {cout<<"invalid" ; return 0;}
      
    }
    cout<<" valid";
}