#include <bits/stdc++.h>
#define int long long
using namespace std;
#ifdef LOCAL
#define dbg(x) cerr << x << "\n";
#else
#define dbg(x)
#endif
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<double> a(n), b(n), prefixa(n), prefixb(n);
for (int i = 0; i < n; i++) {
cin >> a[i] >> b[i];
}
sort(a.begin(), a.end());
sort(b.begin(), b.end());
reverse(a.begin(), a.end());
reverse(b.begin(), b.end());
prefixa[0] = a[0];
prefixb[0] = b[0];
for (int i = 1; i < n; i++) {
prefixa[i] = prefixa[i - 1] + a[i];
prefixb[i] = prefixb[i - 1] + b[i];
}
double best = 0;
int pa = 0;
int pb = 0;
while (pa < n && pb < n) {
double profita = prefixa[pa] - pa - pb - 2;
double profitb = prefixb[pb] - pa - pb - 2;
dbg(profita);
dbg(profitb);
best = max(min(profita, profitb), best);
if (profita < profitb) {
pa++;
} else {
pb++;
}
}
printf("%.4lf\n", (double)best);
}
