18/11/17 12:27:52.98 z9owpr8+.net
>>526 ぺちぷ
<?php
function solve(int $n):int{
$a=0;
for($i=5;$i<=$n;$i*=5)$a+=intdiv($n,$i);
return $a;
}
foreach([1,5,10,1e2,1e6,1e16] as $i)printf("%d -> %d\n",$i,solve($i));
?>
1 -> 0
5 -> 1
10 -> 2
100 -> 24
1000000 -> 249998
10000000000000000 -> 2499999999999996