#include<bits/stdc++.h>

typedef long double db;

db slope(int x1,int y1,int x2,int y2){return 1.0*(y1-y2)/(x1-x2);}

int main(){
	int n,D,H;
	std::cin>>n>>D>>H;
	db min=INFINITY;
	for(int i=0;i<n;++i){
		int d,h;
		std::cin>>d>>h;
		min=std::min(min,slope(d,h,D,H));
	}
	std::cout<<std::max((db)0,H-min*D)<<std::endl;
	return 0;
}
#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main(){
  int n, k, m;
  cin >> n >> k >> m;
  int a, sum=0;
  rep(i, n-1){
    cin >> a;
    sum += a;
  }
  int m2 = m*n;
  int dif = m2 - sum;
  if(dif > k) cout << -1 << endl;
  else if(dif < 0) cout << 0 << endl; 
  else cout << dif << endl;
}