/* package whatever; // don't place package name! */
import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.Scanner;
import java.lang.Math;
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
{
Scanner in
= new Scanner
(System.
in); int m1,m2,m3,x;
m1 = in.nextInt();
m2 = in.nextInt();
m3 = in.nextInt();
if ( ( m1 + m2 + m3 ) % 3 == 0 )
{
if ( m1 != m2 && m1 != m3 && m2 != m3 )
{
System.
out.
print(Math.
max ( Math.
max ( m1 , m2
) , m3
) - ( ( m1
+ m2
+ m3
) / 3 )); }
else if ( m1 == m2 )
{
x = ( m3 - ( m1 + m2 + m3 ) / 3 );
if ( x < 0 )
{
}
}
else if ( m1 == m3 ) {
x = ( m2 - ( m1 + m2 + m3 ) / 3 );
if ( x < 0 )
{
}
}
else if ( m2 == m3 ) {
x = ( m1 - ( m1 + m2 + m3 ) / 3 );
if ( x < 0 )
{
}
}
}
else System.
out.
print("IMPOSSIBLE"); }
}
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