qwq

#include <bits/stdc++.h>

using namespace std ;



int main ()
{
	int l , n ; int a[5200] , b[5200][2] ;
	int minn , maxn , ttime ;
	int zll = -1 , zrr = 5200 ;
	bool qwq = 1 ;
	cin >> l >> n ;
	for ( int i = 1 ; i <= n ; i ++ )
	{
		cin >> a[i] ;
		if ( a[i] >= l / 2 )
		{
			b[i][1] = a[i] ;
			if ( b[i] > zrr ) zrr = i ;
		}
		else
		{
			b[i][0] = a[i] ;
			if ( b[i] > zll ) zll = i ;
		}
	}
	
	while ( qwq )
	{
		for ( int i = 1 ; i <= n ; i ++ )
		{ b[i][0] -- ; b[i][1] ++ ; }
		if ( b[zll] <= 0 && b[zrr] >= l+1 )
		qwq = 0 ;
		minn ++ ;
	}
	cout << minn ;
	
	
	return 0 ;
}