/*
Author-> Codie_1(Akash)
*/
#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define rep(n) for(int i=0;i<n;i++)
#define repi(n) for(int i=0;i<n;i++)
#define repj(n) for(int j=0;j<n;j++)
#define repp(n) for(int i=0;i<=n;i++)
#define repab(a,b) for(int i=a;i<b;i++)
#define w(t) int t; cin>>t; while(t--)
#define endl '\n'
#define pi pair<int, int> p
#define pis pair<int, string> p
#define ff first
#define ss second
#define pb push_back
#define mk make_pair
#define vi vector<int> v
#define vl vector<long long> v;
typedef long long ll;

 

int main()
{
	int n,k,m;
	cin>>n>>k>>m;
	int l=n-1;
	int avg=0;
	while(l--)
	{
		int x;
		cin>>x;
		avg+=x;
	}
	// cout<<avg/(n-1)<<endl;
	int g=(n*m)-avg;
	if(g<=0)
		cout<<0;
	else if(g<=k)
		cout<<g;
	else
		cout<<-1;
	return 0;
}
#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <string>
#include <list>
#include <cassert>
#include <numeric>
#include <cstdint>
#include <queue>
#include <deque>
 
using ll = long long;
using ld = long double;
using namespace std;
using Graph = vector<vector<int>>; 

// typedef pair<int,int> P;
#define teisu 1000000007
#define MAX_WIDTH 60
#define MAX_HEIGHT 60
#define INF 1e9

// int N;
// string S;
// map<int,int> cnt;
// map<char,vector<int>> keep_index;
// ll X,Y,A,B;
// ll ans = 0;
// ll dfs(ll strength,ll keikenti){
//     if(strength >= Y) return 0;

//     ans = max(ans,keikenti);
//     cout << strength << endl;
//     // cout << ans << endl;

//     dfs(strength*A,keikenti+1);

//     dfs(strength+B,keikenti+1);

//     return 0;
// }

double N,D,H,d[110],h[110];
int main(){
    cin >> N >> D >> H;
    for(int i=0;i<N;i++){
        cin >> d[i] >> h[i];
    }

    double ans = 0.000000;
    for(int i=0;i<N;i++){
        
        double calc = h[i] - (((H-h[i])*d[i])/(D-d[i]));
        ans = max(ans,calc);
    }
    // cout << ans << endl;
    ans = min(1000.0000,ans);
    printf("%.15lf\n",ans);

    return 0;
}                            