Vasya came up with his own weather forecasting method. He knows the information about the average air temperature for each of the last n days. Assume that the average air temperature for each day is integral.

Vasya believes that if the average temperatures over the last n days form an arithmetic progression, where the first term equals to the average temperature on the first day, the second term equals to the average temperature on the second day and so on, then the average temperature of the next (n + 1)-th day will be equal to the next term of the arithmetic progression. Otherwise, according to Vasya's method, the temperature of the (n + 1)-th day will be equal to the temperature of the n-th day.

Your task is to help Vasya predict the average temperature for tomorrow, i. e. for the (n + 1)-th day.

Input

The first line contains a single integer n (2 ≤ n ≤ 100) — the number of days for which the average air temperature is known.

The second line contains a sequence of integers t1, t2, ..., tn ( - 1000 ≤ ti ≤ 1000) — where ti is the average temperature in the i-th day.

Output

Print the average air temperature in the (n + 1)-th day, which Vasya predicts according to his method. Note that the absolute value of the predicted temperature can exceed 1000.

Examples

Input

5
10 5 0 -5 -10


Output

-15


Input

4
1 1 1 1


Output

1


Input

3
5 1 -5


Output

-5


Input

2
900 1000


Output

1100

Note

In the first example the sequence of the average temperatures is an arithmetic progression where the first term is 10 and each following terms decreases by 5. So the predicted average temperature for the sixth day is  - 10 - 5 = - 15.

In the second example the sequence of the average temperatures is an arithmetic progression where the first term is 1 and each following terms equals to the previous one. So the predicted average temperature in the fifth day is 1.

In the third example the average temperatures do not form an arithmetic progression, so the average temperature of the fourth day equals to the temperature of the third day and equals to  - 5.

In the fourth example the sequence of the average temperatures is an arithmetic progression where the first term is 900 and each the following terms increase by 100. So predicted average temperature in the third day is 1000 + 100 = 1100.
import java.io.File;import java.io.FileInputStream;import java.io.FileNotFoundException;
import java.io.IOException;import java.io.PrintStream;import java.io.PrintWriter;
import java.security.AccessControlException;import java.util.Arrays;import java.util.Collection;
import java.util.Comparator;import java.util.List;import java.util.Map;import java.util.Objects;
import java.util.Scanner;import java.util.TreeMap;import java.util.function.Function;
import java.util.stream.Collectors;import java.util.stream.IntStream;import java.util.stream.LongStream;
import java.util.stream.Stream;public class _p000847M {static public void main(final String[] args) 
throws IOException{p000847M._main(args);}
static private class p000847M extends Solver{public p000847M(){nameIn="in/1000/p000847M.in";
singleTest=true;}@Override public void solve()throws IOException{int n=sc.nextInt();
sc.nextLine();int[]t=lineToIntArray();int step=2001;for(int i=1;i<n;i++){if(step
==2001){step=t[i]-t[i-1];}else if(step!=t[i]-t[i-1]){step=2002;}}pw.println(step
==2002?t[n-1]:t[n-1]+step);}static public void _main(String[]args)throws IOException
{new p000847M().run();}}static private class Pair<K,V>{private K k;private V v;public 
Pair(final K t,final V u){this.k=t;this.v=u;}public K getKey(){return k;}public 
V getValue(){return v;}}static private abstract class Solver{protected String nameIn
=null;protected String nameOut=null;protected boolean singleTest=false;protected 
boolean preprocessDebug=false;protected boolean doNotPreprocess=false;protected 
PrintStream debugPrintStream=null;protected Scanner sc=null;protected PrintWriter 
pw=null;final static String SPACE=" ";final static String SPACES="\\s+";private 
void process()throws IOException{if(!singleTest){int t=lineToIntArray()[0];while(t--
>0){solve();pw.flush();}}else{solve();pw.flush();}}abstract protected void solve()
throws IOException;protected String[]lineToArray()throws IOException{return sc.nextLine().trim().split(SPACES);
}protected int[]lineToIntArray()throws IOException{return Arrays.stream(lineToArray()).mapToInt(Integer::valueOf).toArray();
}protected long[]lineToLongArray()throws IOException{return Arrays.stream(lineToArray()).mapToLong(Long::valueOf).toArray();
}protected void run()throws IOException{boolean done=false;try{if(nameIn!=null && 
new File(nameIn).exists()){try(FileInputStream fis=new FileInputStream(nameIn);PrintWriter 
pw0=select_output();){done=true;sc=new Scanner(fis);pw=pw0;process();}}}catch(IOException 
ex){}catch(AccessControlException ex){}if(!done){try(PrintWriter pw0=select_output();
){sc=new Scanner(System.in);pw=pw0;process();}}}private PrintWriter select_output()
throws FileNotFoundException{if(nameOut!=null){return new PrintWriter(nameOut);}
return new PrintWriter(System.out);}public static Map<Integer,List<Integer>>mapi(final 
int[]a){return IntStream.range(0,a.length).collect(()->new TreeMap<Integer,List<Integer>>(),
(res,i)->{if(!res.containsKey(a[i])){res.put(a[i],Stream.of(i).collect(Collectors.toList()));
}else{res.get(a[i]).add(i);}},Map::putAll);}public static Map<Long,List<Integer>>
mapi(final long[]a){return IntStream.range(0,a.length).collect(()->new TreeMap<Long,
List<Integer>>(),(res,i)->{if(!res.containsKey(a[i])){res.put(a[i],Stream.of(i).collect(Collectors.toList()));
}else{res.get(a[i]).add(i);}},Map::putAll);}public static<T>Map<T,List<Integer>>
mapi(final T[]a){return IntStream.range(0,a.length).collect(()->new TreeMap<T,List<Integer>>(),
(res,i)->{if(!res.containsKey(a[i])){res.put(a[i],Stream.of(i).collect(Collectors.toList()));
}else{res.get(a[i]).add(i);}},Map::putAll);}public static<T>Map<T,List<Integer>>
mapi(final T[]a,Comparator<T>cmp){return IntStream.range(0,a.length).collect(()->
new TreeMap<T,List<Integer>>(cmp),(res,i)->{if(!res.containsKey(a[i])){res.put(a[i],
Stream.of(i).collect(Collectors.toList()));}else{res.get(a[i]).add(i);}},Map::putAll
);}public static Map<Integer,List<Integer>>mapi(final IntStream a){int[]i=new int[]{0};
return a.collect(()->new TreeMap<Integer,List<Integer>>(),(res,v)->{if(!res.containsKey(v))
{res.put(v,Stream.of(i[0]).collect(Collectors.toList()));}else{res.get(v).add(i[0]);
}i[0]++;},Map::putAll);}public static Map<Long,List<Integer>>mapi(final LongStream 
a){int[]i=new int[]{0};return a.collect(()->new TreeMap<Long,List<Integer>>(),(res,
v)->{if(!res.containsKey(v)){res.put(v,Stream.of(i[0]).collect(Collectors.toList()));
}else{res.get(v).add(i[0]);}i[0]++;},Map::putAll);}public static<T>Map<T,List<Integer>>
mapi(final Stream<T>a,Comparator<T>cmp){int[]i=new int[]{0};return a.collect(()->
new TreeMap<T,List<Integer>>(cmp),(res,v)->{if(!res.containsKey(v)){res.put(v,Stream.of(i[0]).collect(Collectors.toList()));
}else{res.get(v).add(i[0]);}},Map::putAll);}public static<T>Map<T,List<Integer>>
mapi(final Stream<T>a){int[]i=new int[]{0};return a.collect(()->new TreeMap<T,List<Integer>>(),
(res,v)->{if(!res.containsKey(v)){res.put(v,Stream.of(i[0]).collect(Collectors.toList()));
}else{res.get(v).add(i[0]);}},Map::putAll);}public static List<int[]>listi(final 
int[]a){return IntStream.range(0,a.length).mapToObj(i->new int[]{a[i],i}).collect(Collectors.toList());
}public static List<long[]>listi(final long[]a){return IntStream.range(0,a.length).mapToObj(i
->new long[]{a[i],i}).collect(Collectors.toList());}public static<T>List<Pair<T,
Integer>>listi(final T[]a){return IntStream.range(0,a.length).mapToObj(i->new Pair<T,
Integer>(a[i],i)).collect(Collectors.toList());}public static List<int[]>listi(final 
IntStream a){int[]i=new int[]{0};return a.mapToObj(v->new int[]{v,i[0]++}).collect(Collectors.toList());
}public static List<long[]>listi(final LongStream a){int[]i=new int[]{0};return 
a.mapToObj(v->new long[]{v,i[0]++}).collect(Collectors.toList());}public static<T>
List<Pair<T,Integer>>listi(final Stream<T>a){int[]i=new int[]{0};return a.map(v->
new Pair<T,Integer>(v,i[0]++)).collect(Collectors.toList());}public static String 
join(final int[]a){return Arrays.stream(a).mapToObj(Integer::toString).collect(Collectors.joining(SPACE));
}public static String join(final long[]a){return Arrays.stream(a).mapToObj(Long::toString).collect(Collectors.joining(SPACE));
}public static<T>String join(final T[]a){return Arrays.stream(a).map(v->Objects.toString(v)).collect(Collectors.joining(SPACE));
}public static<T>String join(final T[]a,final Function<T,String>toString){return 
Arrays.stream(a).map(v->toString.apply(v)).collect(Collectors.joining(SPACE));}public 
static<T>String join(final Collection<T>a){return a.stream().map(v->Objects.toString(v)).collect(Collectors.joining(SPACE));
}public static<T>String join(final Collection<T>a,final Function<T,String>toString)
{return a.stream().map(v->toString.apply(v)).collect(Collectors.joining(SPACE));
}public static<T>String join(final Stream<T>a){return a.map(v->Objects.toString(v)).collect(Collectors.joining(SPACE));
}public static<T>String join(final Stream<T>a,final Function<T,String>toString){
return a.map(v->toString.apply(v)).collect(Collectors.joining(SPACE));}public static
<T>String join(final IntStream a){return a.mapToObj(Integer::toString).collect(Collectors.joining(SPACE));
}public static<T>String join(final LongStream a){return a.mapToObj(Long::toString).collect(Collectors.joining(SPACE));
}public static List<Integer>list(final int[]a){return Arrays.stream(a).mapToObj(Integer::valueOf).collect(Collectors.toList());
}public static List<Integer>list(final IntStream a){return a.mapToObj(Integer::valueOf).collect(Collectors.toList());
}public static List<Long>list(final long[]a){return Arrays.stream(a).mapToObj(Long::valueOf).collect(Collectors.toList());
}public static List<Long>list(final LongStream a){return a.mapToObj(Long::valueOf).collect(Collectors.toList());
}public static<T>List<T>list(final Stream<T>a){return a.collect(Collectors.toList());
}public static<T>List<T>list(final T[]a){return Arrays.stream(a).collect(Collectors.toList());
}}}
