#include <iostream>
#include <algorithm>
using namespace std;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
double s0, s;
double A[n], B[n];
cin >> s0 >> s;
if(s0>=s)
cout << 0;
else
{
for(int i=0; i<n; i++)
{
cin >> A[i];
B[i]=A[i];
}
sort(B, B+n);
int ans=0;
for(int i=n-1; i>=1; i--)
{
if(B[i]>=10.1)
{
if((int)(B[i]-5.1)+s0>=s){
ans++;
cout << ans;
return 0;
}
else
{
ans++;
B[i-1]+=(int)(B[i]-5.1);
}
}
else
{
break;
}
}
if((B[0]>=10.1)&&((int)(B[0]-5.1)+s0>=s))
cout << ans+1;
else
{
sort(A, A+n);
if(s0>=10.1)
{
A[n-1]+=(int)(s0-5.1);
s0-=(int)(s0-5.1);
s0-=0.1;
ans=1;
for(int i=n-1; i>=1; i--)
{
if(A[i]>=10.1)
{
if((int)(A[i]-5.1)+s0>=s){
ans++;
cout << ans;
return 0;
}
else
{
ans++;
A[i-1]+=(int)(A[i]-5.1);
}
}
else
{
break;
}
}
if((A[0]>=10.1)&&((int)(A[0]-5.1)+s0>=s))
cout << ans+1;
else
cout << -1;
}
else
cout << -1;
}
}
return 0;
}
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