#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

void encrypt(FILE * f, int key) {    
  int c;    
  while ((c = fgetc(f)) != EOF) {        
    if (isalpha(c)) {            
      c = tolower(c);            
      c -= 'a';            
      c += key;            
      c %= 26;            
      c += 'a';        
    }        
    printf("%c", c);    
  }
}

int main(int argc, char ** argv) {  
  if (argc != 3) {    
    fprintf(stderr,"Usage: encrypt key inputFileName\n");    
    return EXIT_FAILURE;  
  }  
  int key = atoi(argv[1]);  
  if (key == 0) {    
    fprintf(stderr,"Invalid key (%s): must be a non-zero integer\n", argv[1]);    
    return EXIT_FAILURE;  
  }  
  FILE * f = fopen(argv[2], "r");  
  if (f == NULL) {    
    perror("Could not open file");    
    return EXIT_FAILURE;  
  }  
  encrypt(f,key);  
  if (fclose(f) != 0) {    
    perror("Failed to close the input file!");    
    return EXIT_FAILURE;  
  }  
  return EXIT_SUCCESS;
}