#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
using vl = vector<ll>;

/* short */
#define pb push_back
#define mp make_pair
#define Fi first
#define Se second
#define ALL(v) begin(v), end(v)
#define RALL(v) rbegin(v), rend(v)

/* REPmacro */
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define FORR(i, a, b) for(int i = (a); i >= (b); i--)
#define REP(i, n) for(int i = 0; i < (n); i++)
#define FOREACH(x, a) for(auto x : a)

/* exchange */
#define CHMIN(a, b) (a) = min((ll)(a), (ll)(b))
#define CHMAX(a, b) (a) = max((ll)(a), (ll)(b))

/* function */
#define IN(x) cin >> x
#define DEBUG(x) cerr << (x) << " "
#define LN() cerr << "\n"
#define PRINT(x) cout << (x) << endl
#define BR cout << endl

/* const */
const int ARRAY = 100005;
const int INF = 1001001001; // 10^9
const ll LINF = 1001001001001001001; // 10^18
const int MOD = 1e9 + 7;

ll N = 0;
ll K = 0;
ll M;
ll t;
ll ret = 0;
string s;
ll total = 0;

void input() {
  IN(N);
  IN(K);
  IN(M);
  REP(i, N-1) {
    IN(t);
    total += t;
  }
}

void solve() {
  ll goal = N * M;
  if (goal - total < 0) {
    PRINT(0);
  } else if (goal - total > K) {
    PRINT(-1);
  } else {
    PRINT(goal - total);
  }
}

int main(void){
  input();
  solve();
}

#include<bits/stdc++.h>
using namespace std;
#define lli long long int
#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define FORD(i,a,b) for(int i=(a);i>=(b);i--)
inline bool EQ(double a, double b) { return fabs(a-b) < 1e-9; }
const int INF = 1<<29;
typedef long long ll;
inline int two(int n) { return 1 << n; }
inline int test(int n, int b) { return (n>>b)&1; }
inline void set_bit(int & n, int b) { n |= two(b); }
inline void unset_bit(int & n, int b) { n &= ~two(b); }
inline int last_bit(int n) { return n & (-n); }
inline int ones(int n) { int res = 0; while(n && ++res) n-=n&(-n); return res;}

int main()
{
	lli n,k,m;
	cin>>n>>k>>m;
	lli avg=0;
	for(int i=1;i<n;i++)
	{lli x;
		cin>>x;
		avg+=x;}
	lli ans=n*m-avg;
	if(ans<=0)
		cout<<0;
	else if(ans<=k)
		cout<<ans;
	else
		cout<<-1;


}

