<?php
$n = trim(fgets(STDIN));
$numstr = trim(fgets(STDIN));
$tail = strlen($numstr);
for(;$n;$n--) {
    $pos = strpos($numstr, (string)$n);
    if($pos > $tail) {
        break;
    }
    $tail = $pos;
}
echo $n;
echo PHP_EOL;