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

int main(){
    ll n,k,m;
    cin >> n >> k >>m;
    int i,j;
    ll yotei = m*n;
    for(i=0;i<n-1;i++){
        ll ten;
        cin >> ten;
        yotei-=ten;
    }
    if(yotei<=0)cout << 0 <<endl;
    else if(yotei>k)cout << -1 << endl;
    else cout << yotei << endl;

    return 0;
 }

#include <bits/stdc++.h>
typedef long long int   LL;
typedef long double     LD;
#define nl              '\n'
#define ff              first
#define ss              second
#define inf             INT_MAX
#define SQR(a)          (a) * (a)
#define pb              push_back
#define GCD(a, b)       __gcd(a, b)
#define PI              2.0*acos(0.0)
#define pii             pair<int,int>
#define pll             pair<long long,long long>
#define LCM(a, b)       (a * b) / GCD(a, b)
#define mem(a,b)        memset(a,b,sizeof(a))
#define srtv(v)         sort(v.begin(),v.end())
#define Rep(i,a,b)      for(int i = a; i <= b; i++)
#define rep(i,a,b)      for(int i = a; i >= b; i--)
#define FOR(i,a)        for(int i=0;i<a;i++)
//#pragma                 GCC optimize("Ofast,no-stack-protector")
//#pragma                 GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma                 GCC optimize("unroll-loops")
#define boost           ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define inout           freopen("input.txt","r",stdin);freopen("output.txt","w",stdout)

#define si(x)           scanf("%d",&x)
#define pi(x)           printf("%d",x)
#define sss(str)         scanf("%s",str)
#define pl(x)           printf("%lld",x)
#define sl(x)           scanf("%lld",&x)
#define sii(x,y)        scanf("%d %d",&x,&y)
#define sll(x,y)        scanf("%lld %lld",&x,&y)
#define siii(x,y,z)     scanf("%d %d %d",&x,&y,&z)
#define slll(x,y,z)     scanf("%lld %lld %lld",&x,&y,&z)
using namespace std;
int dx[]={0,0,-1,1};
int dy[]={1,-1,0,0};


//--------------------------code starts here---------------------------------------
const int maxn=1e5+5;


void solve(int test)
{
    LL a,b,c,d;
    cin>>a>>b>>c>>d;
    int cnt=0;
    LL r=0;
    Rep(i,1,1e6)
    {
        if(a<=d*r)
        {
            break;
        }

        a+=b;
        r+=c;
        cnt++;
    }

    if(cnt==1e6)
        cnt=-1;

    cout<<cnt<<nl;
}

int main()
{
	int test=1;
    //si(test);
    Rep(i,1,test)
    {
        solve(test);
    }

	return 0;
}