#include <bits/stdc++.h>
using namespace std;
long long read() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (ch - '0' < 0 || ch - '0' > 9) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch - '0' >= 0 && ch - '0' <= 9) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int n, K, m;
int ans;
int main() {
  n = read();
  K = read();
  m = read();
  for (int i = 1; i <= n - 1; i++) {
    ans += read();
  }
  ans = n * m - ans;
  if (ans <= K)
    cout << max(ans, 0) << endl;
  else
    puts("-1");
  return 0;
}
