using System;
using System.IO;
class Start{
  static void Solve(){
    long k,n,f;Inl(out k,out n,out f);
    long[] a=new long[f];
    Inl(ref a);
    long sum=0;
    for(int i=0;i<f;i++)sum+=a[i];
    Outl(Max(-1,k*n-sum));
  }
  
  static void Main(){
  	var sw=new StreamWriter(Console.OpenStandardOutput()){AutoFlush=false};
    Console.SetOut(sw);
    Solve();
    Console.Out.Flush();
  }
  
  static void Inl(out long t){t=long.Parse(Console.ReadLine());}
  static void Inl(out string t){t=Console.ReadLine();}
  static void Inl(out char t){t=Console.ReadLine()[0];}
  static void Inl(out double t){t=double.Parse(Console.ReadLine());}
  static void Inl<T>(ref T[] t){
    string[] s=Console.ReadLine().Split();
    Type type=typeof(T);
    int n=s.Length;
    if(type==LONG)for(int i=0;i<n;i++)t[i]=(T)(object)long.Parse(s[i]);
    else if(type==STRING)for(int i=0;i<n;i++)t[i]=(T)(object)s[i];
    else if(type==CHAR)for(int i=0;i<n;i++)t[i]=(T)(object)s[i][0];
    else for(int i=0;i<n;i++)t[i]=(T)(object)double.Parse(s[i]);
  }
  static void Inl<T,U>(out T t,out U u){string[] s=Console.ReadLine().Split();In(out t,ref s,0);In(out u,ref s,1);}
  static void Inl<T,U,V>(out T t,out U u,out V v){string[] s=Console.ReadLine().Split();In(out t,ref s,0);In(out u,ref s,1);In(out v,ref s,2);}
  static void Inl<T,U,V,W>(out T t,out U u,out V v,out W w){string[] s=Console.ReadLine().Split();In(out t,ref s,0);In(out u,ref s,1);In(out v,ref s,2);In(out w,ref s,3);}
  static void Inl<T,U,V,W,X>(out T t,out U u,out V v,out W w,out X x){string[] s=Console.ReadLine().Split();In(out t,ref s,0);In(out u,ref s,1);In(out v,ref s,2);In(out w,ref s,3);In(out x,ref s,4);}
  static void Inl<T,U,V,W,X,Y>(out T t,out U u,out V v,out W w,out X x,out Y y){string[] s=Console.ReadLine().Split();In(out t,ref s,0);In(out u,ref s,1);In(out v,ref s,2);In(out w,ref s,3);In(out x,ref s,4);In(out y,ref s,5);}
  static void Inl<T,U,V,W,X,Y,Z>(out T t,out U u,out V v,out W w,out X x,out Y y,out Z z){string[] s=Console.ReadLine().Split();In(out t,ref s,0);In(out u,ref s,1);In(out v,ref s,2);In(out w,ref s,3);In(out x,ref s,4);In(out y,ref s,5);In(out z,ref s,6);}
  static void In<T>(out T t,ref string[] s,int i){
    Type type=typeof(T);
    if(type==LONG)t=(T)(object)long.Parse(s[i]);
    else if(type==STRING)t=(T)(object)s[i];
    else if(type==CHAR)t=(T)(object)s[i][0];
    else t=(T)(object)double.Parse(s[i]);
  }
  static Type LONG=typeof(long);
  static Type STRING=typeof(string);
  static Type CHAR=typeof(char);

  static void Out<T>(T t){Console.Write(t);}
  static void Outl<T>(T t){Console.WriteLine(t);}
  static void Out(double d,int n){string s=d.ToString("F"+n);Console.Write(s);}
  static void Outl(double d,int n){string s=d.ToString("F"+n);Console.WriteLine(s);}

  static void Swap<T>(ref T a,ref T b){T c=a;a=b;b=c;}
  static void Sort<T>(ref T[] t){Array.Sort(t);}
  static T Max<T>(params T[] t)where T:IComparable{T max=t[0];for(int i=1;i<t.Length;i++)if(max.CompareTo(t[i])<0)max=t[i];return max;}
  static T Min<T>(params T[] t)where T:IComparable{T max=t[0];for(int i=1;i<t.Length;i++)if(max.CompareTo(t[i])>0)max=t[i];return max;}
}