package src;

import java.util.*;
class B{
	static Scanner s = new Scanner(System.in);
	public static void main(String[] args) {
		long[] out = new long[s.nextInt()+1];
		out[0]=1;
		long buf=1;
		boolean f=false;
		for(int i=1;i<out.length;++i) {
			if(i%2==0) {
				buf=(out[i]=buf-Long.parseLong(s.next()));
			}else {
				buf=(out[i]=Long.parseLong(s.next())-buf);
			}
			f|=(buf<=0);
		}
		/*
		if(f)
			System.out.println(-1);
		else {
			System.out.println(out.length);
			for(int i=0;i<out.length;i++)
				System.out.println(out[i]);
		}
		*/
	}
}