using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using static System.Console;

public class hello{
    public static void Main(){ //yukicorder No.143 豆
             //一袋K粒 N袋 家族はF人        
        var s = ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
        var k = s[0];
        var n = s[1];
        var f = s[2];
        var t =  ReadLine().Trim().Split(' ').Select(int.Parse).ToArray();
        
        var beans = k*n;
        int sum = t.Sum();
        int sa = beans - sum;
        
        if(sa<0) {
            WriteLine(-1);
        }else{
            WriteLine(sa);
        }
    }
}