i am getting error in this c program

i am trying to implement this c program , i want to expand strings eg a-z to a b c d ...z etc when i put the input string as a-z i dont get the o/p but when i use the input string as az or without using the - i get the desired o/p

#include<stdio.h>
main()
{
    char s1[20] = "a   z", s2[10], s3[30];
    int k = 0, i = 0;
    while (s1[i] != '\0')
    {
        if((s1[i] >= 65 && s1[i]<= 90) || (s1[i] >= 97 && s1[i]<= 122))
            s2[k]=s1[i];
        i++;  k++;
    }
    s2[k] = '\0';
    for (int m = s2[0] ; m <= s2[1] ; m++)
       printf("%c ",m);
}