#include <bits/stdc++.h>
using namespace std;
#define ll unsigned long long int
#define mod 1000000007
bool isPrime[1000001];
ll power(ll a,ll b)
{
    ll result = 1;
    while(b>0)
    {
        if(b%2)
        {
            result = (a*result)%mod;
        }
        a = (a*a)%mod;
        b = b/2;
    }
    return result;
}
int gcd(int a,int b)
{
    if(b==0)
    {
        return a;
    }
    return gcd(b,a%b);
}
void sieve()
{
    memset(isPrime,1,sizeof(isPrime));
    isPrime[0] = isPrime[1] = 0;
    for(int i=2;i*i<=1000000;i++)
    {
        if(isPrime[i])
        {
            for(int j=i*i;j<=1000000;j+=i)
            {
                isPrime[j] = 0;
            }
        }
    }
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL),cout.tie(NULL);
    int t,n;
    t = 1;
    for(int i=0;i<t;i++)
    {
        int n,k,m;
        cin>>n>>k>>m;
        int req = n*m;
        int sum =0,x;
        for(int i=0;i<n-1;i++)
        {
            cin>>x;
            sum+=x;
        }
        int ans = max(req-sum,0);
        if(ans>k)
        {
            cout<<"-1";
        }
        else
        {
            cout<<ans;
        }
    }
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

int main() {
  int n,k,m,a,x;
  cin >> n >> k >> m;

  int sum = 0;
  for(int i = 0; i < (n -1); i++){
    cin >> a;
    sum += a;
  }
  
  x = m * n - sum;
  x = 0 > x ? 0 : x; 
  x = k >= x ? x : -1 ;
  
  cout << x << endl;    
}