#!/usr/local/bin/perl

while ($line = <STDIN>) {
        $line =~ tr/A-Z/a-z/;
        $line =~ s/[^a-z]//g;
        @letters = split(//, $line);
        foreach $letter (@letters) {
                $lettercount{$letter} += 1;
        }
}

$~ = "WRITEHEADER";
write;
$count = 0;
foreach $letter (reverse sort occurrences
                (keys(%lettercount))) {
        &write_letter($letter, $lettercount{$letter});
        last if (++$count == 5);
}

sub occurrences {
        $lettercount{$a} <=> $lettercount{$b};
}
sub write_letter {
        local($letter, $value) = @_;

        $~ = "WRITELETTER";
        write;
}
format WRITEHEADER =
The five most frequently occurring letters are:
.
format WRITELETTER =
        @:  @<<<<<<
        $letter, $value
.
