#!/usr/bin/env tclsh

if {[llength $argv]} {
  set pre [lindex $argv 0]
} else {
  set pre 25
}

while {[gets stdin line] >= 0} {
  if {$line eq ""} continue
  lappend list $line
}

proc check {list i} {
  global pre
  for {set j [expr {$i-$pre}]} {$j < $i-1} {incr j} {
    for {set k [expr {$j+1}]} {$k < $i} {incr k} {
      set a [lindex $list $j]
      set b [lindex $list $k]
      if {$a == $b} continue
      if {$a + $b == [lindex $list $i]} {return 1}
    }
  }
  return 0
}

for {set i $pre} {$i < [llength $list]} {incr i} {
  if {! [check $list $i]} {
    puts [lindex $list $i]
    exit
  }
}