<?php
function getStandardInput()
{
  $array = array();
  while (true) {
    $stdin = fgets(STDIN);
    if ($stdin == "") {
      break;
    }
    $array[] = rtrim($stdin);
  }
  return $array;
}

function splitBySpace($str)
{
  return explode(' ', $str);
}

function bubbleSort($array)
{
    $cnt = count($array);

    for($i=0;$i<$cnt-1;$i++){
        for($j=$i+1;$j<$cnt;$j++){
            if($array[$i]>$array[$j]){
                $tmp = $array[$i];
                $array[$i] = $array[$j];
                $array[$j] = $tmp;
            }
        }
    }
    return $array;
}

$stdIn = getStandardInput();
$splitStdIn1 = splitBySpace($stdIn[0]);
$beanCount = $splitStdIn1[0];
$packCount = $splitStdIn1[1];
$familyCount = $splitStdIn1[2];
$remainBeanCount = $beanCount * $packCount;

$familyAges = splitBySpace($stdIn[1]);

foreach ($familyAges as $familyAge){
    $remainBeanCount = $remainBeanCount - $familyAge;
}

if($remainBeanCount < 0){
    echo(-1);
} else {
    echo($remainBeanCount);
}