7 => 7
1000 => 819
3456 => 2562
10000 => 7380

f(int n)
{
    int i=1;
    int c=n;

    for(; i<=n; i++)
    {
        int m=-1;
        int x=i;

        while(x)
        {
            if (x % 10 == m)
            {
                c--;
                break;
            }
            m = x % 10;
            x /= 10;
       }
   }
   return c;
}