#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<queue>

using namespace std;
#define int long long
#define rep(i,n) for(int i=0;i<n;++i)
#define rep1(i,n) for(int i=1;i<=n;++i)
signed main(){
  int n,k,m;cin>>n>>k>>m;
  vector<int> a(n,0);
  rep(i,n-1)cin>>a[i];
  int sum=0;
  rep(i,n-1){
    sum+=a[i];
  }
  bool tag=false;
  int ans=n*m-sum;
  if(ans>k){
    tag=true;
  }
  if(ans<0)ans=0;
  if(tag)ans=-1;
  cout<<ans<<endl;
  return 0;
}
#include <iostream>
using namespace std;
int main()
{
	int n,k,m,x,y=0,z;
	cin>>n>>k>>m;
	int a[100];
	x=m*n;
	for(int i=0; i<n-1; i++)
	{
		cin>>a[i];
		y=y+a[i];
	}
	z=x-y;
	if(k>=z && z>0)
	{
	cout<<z<<endl;
	}else if(k<z){
		cout<<"-1"<<endl;
	}else{
		cout<<"0"<<endl;
	}
}