#include<bits/stdc++.h>
using namespace std;
#define fastIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define int long long int
#define fi first
#define se second
#define pub push_back
#define pi pair<int,int>
#define all(v) (v).begin(), (v).end()
#define rep(i, l, r) for(int i=(int)(l);i<(int)(r);i++)
#define repd(i, l, r) for (int i=(int)(l);i>=(int)(r);i--)
#define clrg(i, l, r) for(int i=(int)(l);i<(int)(r);i++)vis[i]=0,v[i].clear();
int power(int x, unsigned int y){int res = 1;while (y > 0){ if (y & 1){res = res*x;} y = y>>1;x = x*x;}return res;}
int powermod(int x, unsigned int y, int p){int res = 1;x = x % p;while (y > 0){if (y & 1){res = (res*x) % p;}y = y>>1; x = (x*x) % p;}return res;}
#define print2d(mat,n,m){for(int i=0;i<(int)(n);i++){for(int j=0;j<(m);j++){cout<<mat[i][j]<<" ";}cout<< endl;}}
#define clr(a,x) memset(a,x,sizeof(a))
#define rr(v) for(auto &val:v)
#define print(v) for (const auto itr : v){cout<<itr<<' ';}cout<<"\n";
#define ln length()
#define sz size()
#define mod 1000000007
#define elif else if
#define ld long double
int32_t main(){
   fastIO
   int n,d,h; cin>>n>>d>>h;
   ld dd=d,hh=h;
   vector<int> hs,ds;
   ld ans=0.0;
   rep(i,0,n){
       ld hi,di; cin>>di>>hi;
       ld x=dd*(hh-hi)/(dd-di);
       ans=max(ans,hh-x); 
   }
   cout<<fixed<<setprecision(5);
    cout<<ans<<"\n";
   return 0;
}

/*  *Which of the favors of your Lord will you deny ?*  */
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define vpnt(ans)                       \
    for (ll i = 0; i < ans.size(); i++) \
        cout << ans[i] << (i + 1 < ans.size() ? ' ' : '\n');
#define setbit(x, k) (x |= (1LL << k))
#define clearbit(x, k) (x &= ~(1LL << k)) //x er last theke k-1 tomo ghor
#define checkbit(x, k) (x & (1LL << k))
#define mp make_pair
#define scl(x) scanf("%lld", &x)
#define sci(x) scanf("%d", &x)
#define pb push_back
#define pf push_front
#define ppb pop_back
#define ppf pop_front
#define ff first
#define ss second
#define pii pair<int, int>
#define YES printf("YES\n")
#define Yes printf("Yes\n")
#define yes printf("yes\n")
#define NO printf("NO\n")
#define No printf("No\n")
#define no printf("no\n")
#define nn cout << "\n";
#define INF (1 << 30)
#define LL_INF (1LL << 32)
#define pll pair<ll, ll>
#define pp(n, p) (ll)(pow(n, p) + 0.5)
#define ok ios::sync_with_stdio(false);
#define fine cin.tie(nullptr);
#define mod 1000000007
#define N 505

int main()
{
    int n, m;
    cin>>n>>m;
    int a[m+1];
    for(int i=0; i<m; i++) cin>>a[i];
    sort(a, a+m);
    int p = 0;
    int mn = INF;
    vector<int>v;
    for(int i=0; i<m; i++) 
    {
        int d = a[i]-p-1;
        if(d)
        {
            mn = min(d, mn);
            v.pb(a[i]-p-1);
        }
        p = a[i];
    }
    if(n-p>0) mn = min(n-p, mn);
    v.pb(n-p);
    
    if(mn==0) {cout<<0;nn; return 0;}
    int res = 0;
    for(int i=0; i<v.size(); i++)
    {
        res += ceil((double) v[i]/mn);
    }
    cout<<res;nn;
}