#define NguyenDangQuan the_author
#define my_heart love_you_to_the_moon_and_back

#include <bits/stdc++.h>
#define pb push_back
#define ld long double
#define ll long long
#define ull unsigned long long
#define float double
using namespace std;

int typetest;
inline void fastIOfileinput(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    typetest = 0;
}

const int N = 1e5 + 3;
int n;
using floats = float*;
float a[N], b[N], c[N];

bool lef(const float& a, const float &b){
    return a > b;
}

void Enter(){
    cin >> n;
    for(int i = 1; i <= n; ++i){
        cin >> a[i] >> b[i];
    }
    sort(a + 1, a + n + 1, lef);
    sort(b + 1, b + n + 1, lef);
}

void sub1(){
    float ans = 0;
    for(int i = 1; i <= n; ++i)
        for(int j = 1; j <= n; ++j)
            ans = max(ans, min(a[i] - i - j, b[j] - i - j));
    cout << ans;
}

void sub2(){
    int minn[N];
    float ans = 0;
    minn[1] = 1;
    for(int i = 2; i <= n; ++i)
        if(c[i] > c[minn[i - 1]])
            minn[i] = i;
        else
            minn[i] = minn[i - 1];
    for(int i = 1; i <= n; ++i){
        int piv = min(n, int(a[i] - i));
        int l = 0, m, h = piv;
        while(l <= h){
            m = (l + h) / 2;
            if(b[m] < a[i]) l = m + 1;
            else h = m - 1;
        }
        if(h > 0)
            ans = max(ans, c[minn[h]] - i);
        if(l <= piv)
            ans = max(ans, a[i] - i - l);
    }
    cout << ans;
}

void solve(){
    for(int i = 1; i <= n; ++i){
        a[i] += a[i - 1];
        b[i] += b[i - 1];
        c[i] = b[i];
        c[i] -= i;
    }
    cout << fixed << setprecision(4);
    //if(n <= 1000)
    //  sub1();
    //else
        sub2();
}

main(){
    fastIOfileinput();
    if(typetest){
        int t;
        cin >> t;
        while(t--){
            Enter();
            solve();
        }
    }
    else{
        Enter();
        solve();
    }
}