Problem:
Score : 100 points

Problem Statement

Given is a lowercase English letter C that is not z. Print the letter that follows C in alphabetical order.

Constraints

C is a lowercase English letter that is not z.

Input

Input is given from Standard Input in the following format:

C

Output

Print the letter that follows C in alphabetical order.

Sample Input 1

a

Sample Output 1

b

a is followed by b.

Sample Input 2

y

Sample Output 2

z

y is followed by z.

Solution in PHP7 (7.0.15) (judge verdict: Wrong Answer)

<?php
define("MOD", 10**9+7);
define("MAX", 10**5);
fscanf(STDIN, "%d%d", $n, $k);
$A = array_map("intval", explode(" ", fgets(STDIN)));
sort($A);

$f[0] = $invf[0] = $invf[1] = 1;
for($i = 1; $i <= $n; $i++)$f[$i] = $f[$i-1]*$i%MOD;
for($i = 2; $i <= $n; $i++)$invf[$i] = (MOD - intdiv(MOD, $i))*$invf[MOD%$i]%MOD;
for($i = 2; $i <= $n; $i++)$invf[$i] = $invf[$i]*$invf[$i-1]%MOD;;
$ans = 0;
foreach($A as $i => $a){
    if($n-$i-1 >= $k-1){
        $ans = ($ans + MOD*MOD - $a * nCm($n-$i-1, $k-1))%MOD;
    }
}
$A = array_reverse($A);
foreach($A as $i => $a){
    if($n-$i-1 >= $k-1){
        $ans = ($ans + $a * nCm($n-$i-1, $k-1))%MOD;
    }
}
echo (($ans%MOD)+MOD)%MOD;
function nCm($n, $m){
    global $f, $invf;
    return $f[$n] * $invf[$n-$m] % MOD * $invf[$m] % MOD;
}