C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	long long n,m,s,i,x,r;
	cin>>n>>m;
	s = 0;
	
	//m次移动
	for(i = 1;i <= m;i++){
		cin>>x;
		s = s + x; 
	}
	
	//去掉完整的若干来回，实际还需要走几个格子
	s = s % ((n - 1) * 2); 
	//如果是向右能走完
	if(s <= n - 1){
		r = 1 + s;
	} else{
		r = n - s % (n - 1);
	}
	cout<<r<<endl;
	
	return 0;
}