Hi, please, tell me
Why do my task solve number 1404 take WA#1:

#include <stdio.h>

char Change(char ch);

int main()
{
 #ifndef ONLINE_JUDGE
  freopen("input.txt", "rt", stdin);
  freopen("output.txt", "wt", stdout);
 #endif

 char ch;
 scanf("%c", &ch);
 while (!feof(stdin))
 {
  ch=ch-'a';
  printf("%c", Change(ch)+'a');
  scanf("%c", &ch);
 }
 return 0;
}

char Change(char ch)
{
 static int s=5;
 int ch_res=ch-s;
 while (ch_res<0)
 {
  ch_res+=26;
 }
 s+=ch_res;
 return ch_res;
}

This program output: "secret" on first test from task's condition, but take WA#1