/* Bismillahirrahmanirrahim */
/* Author : Jehian Norman Saviero (@Reiva5) */
#include <bits/stdc++.h>
 
// MACROS SAMPAH
#define Jehian using
#define Norman namespace
#define Saviero std
 
Jehian Norman Saviero;
 
// MACROS TYPE
typedef long l;
typedef long long ll;
typedef unsigned long ul;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pi;
typedef pair<l,l> pl;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<l> vl;
typedef vector<ll> vll;
 
// MACROS FOR PRINT
#define nl printf("\n")
 
// MACROS FOR ATTRIBUTE
#define fi first
#define se second
 
// MACROS FOR FUNCTION AND METHOD
#define eb emplace_back
#define mp make_pair
#define pb push_back
#define pf push_front
#define ppb pop_back
#define ppf pop_front
#define ALL(x) (x).begin(), (x).end()
#define ALLN(x,n) (x), (x)+(n)
#define CUBE(x) (SQR(x)*(x))
#define SET(x,y) for (int _qwe = sizeof(x)/sizeof(x[0]); _qwe; ) x[--_qwe] = (y)
#define SQR(x) ((x)*(x))
#define SSORT(x,sz) sort(ALLN(x,sz))
#define RESET(x) memset((x), 0, sizeof(x))
#define VSORT(x) sort(ALL(x))
 
// MACROS READ INPUT
#define sci(x) scanf("%d", &x)
#define scl(x) scanf("%ld", &x)
#define scll(x) scanf("%lld", &x)
#define scd(x) scanf("%lf", &x)
#define scld(x) scanf("%Lf", &x)
#define scul(x) scl(x)
#define scull(x) scll(x)

int main() {
	int N, K, M;
	sci(N), sci(K), sci(M);
	ll sum = 0;
	for (int i = 0; i < N-1; ++i) {
		int curr;
		sci(curr);
		sum += curr;
	}
	ll tot = M * N;
	ll diff = tot - sum;
	if (0 <= diff && diff <= K) {
		printf("%lld\n", diff);
	} else if (diff < 0) {
		printf("0\n");
	} else {
		printf("-1\n");
	}
	return 0;
}
#include <bits/stdc++.h>
using namespace std;

int main() {
  int n, k, m;
  cin >> n >> k >> m;
  int s = 0;
  int A;
  for(int i = 0; i < n - 1; i++){
    cin >> A;
    s += A;
  }
  int B = n*m - s;
  if(B > k){
    cout << -1;
  }
  else if(B < 0){
    cout << 0;
  }
  else{
    cout << B;
  }
}