using System;
using System.Diagnostics;

namespace CodeIq
{
	internal class Program
	{
		private static void Main()
		{
			// パラメータ取得
			var line1 = Console.ReadLine();
			if( string.IsNullOrEmpty( line1 ) )
				return;
			var line2 = Console.ReadLine();
			if( string.IsNullOrEmpty( line2 ) )
				return;

			var paramData = line1.Split( ' ' );
			var k = int.Parse( paramData[0] );
			var n = int.Parse( paramData[1] );
			var beanCount = k * n;

			var eatBeanCount = 0;
			foreach( var age in line2.Split( ' ' ) )
				eatBeanCount += int.Parse( age );

			var result = beanCount - eatBeanCount;
			Debug.Print( ( result >= 0 ? result : -1 ).ToString() );
			Console.WriteLine( result >= 0 ? result : -1 );
		}
	}
}