#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ull unsigned long long
#define ld long double
#define pii pair<int, int>
#define fi first
#define se second
#define __builtin_popcount __builtin_popcountll
#define all(x) (x).begin(), (x).end()
#define BIT(x, i) (((x) >> (i)) & 1)
#define MASK(x) ((1ll << (x)))
#define debug(a, l, r) for (int _i = (l); _i <= (r); ++_i) cout << (a)[_i] << ' '; cout << '\n';
const int MAXN = 1e5 + 5;
int n;
signed main() {
#ifdef NCTHANH
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(0);
cin.tie(nullptr); cout.tie(nullptr);
cin >> n;
priority_queue<ld> pqa, pqb;
for (int i = 1; i <= n; ++i) {
ld a, b; cin >> a >> b;
pqa.push(a); pqb.push(b);
}
ld suma = 0, sumb = 0, res = 0;
int iter = 0;
while (!pqa.empty() || !pqb.empty()) {
if (pqa.empty() || !(suma < sumb)) {
ld cur = pqb.top(); pqb.pop();
// cout << cur << '\n';
sumb += cur;
}
else if (pqb.empty() || suma < sumb) {
ld cur = pqa.top(); pqa.pop();
// cout << cur << '\n';
suma += cur;
}
++iter;
// if (iter == 10) break;
res = max(res, min(suma, sumb) - (ld)iter);
}
cout << fixed << setprecision(4);
cout << res;
return 0;
}
