# 题解：

#pragma GCC optimize(3)
#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for (int i = 0; i < (n); i++)
#define Rep(i, n) for (int i = 1; i <=(n); i++)
#define range(x) begin(x), end(x)

typedef complex<double> pt, vec;
typedef pair<pt, pt> seg;

const double EPS = 1e-8;
int fcmp(double a, double b = 0.0) {
if (fabs(a - b) < EPS) return 0;
return a < b ? -1 : 1;
}

double operator * (pt a, pt b) {
return a.real() * b.imag() - a.imag() * b.real();
}

double operator , (pt a, pt b) {
return a.real() * b.real() + a.imag() * b.imag();
}

inline bool ptOnSeg(pt p, seg s){
vec v1 = s.first - p, v2 = s.second - p;
return fcmp((v1, v2)) <= 0 && fcmp(v1 * v2) == 0;
}

inline pt getIntersection(pt P, vec v, pt Q, vec w) {
return P + v * (w*(P-Q)/(v*w));
}

// -1 outside the polygon
// 0  on the border of the polygon
// 1  inside the polygon
int ptOnPoly(pt p, pt* poly, int n){
//  fprintf(stderr, "(%.7f, %.7f)\n", p);
int wn = 0;
for (int i = 0; i < n; i++){
if (ptOnSeg(p, {poly[i], poly[i+1]})) return 0;
int d1 = fcmp(poly[i].imag() - p.imag()),
d2 = fcmp(poly[i+1].imag() - p.imag()),
k = fcmp((poly[i+1] - poly[i])*(p - poly[i]));
if (k > 0 && d1 <= 0 && d2 > 0) wn++;
if (k < 0 && d2 <= 0 && d1 > 0) wn--;
}
return wn ? 1 : -1;
}

istream& operator >> (istream& lhs, pt& rhs){
double x, y;
lhs >> x >> y;
rhs = {x, y};
return lhs;
}

int n;
vector<pt> poly;
double ans = 0.0;

void attempt(pt a, pt b) {
vector<pt> points;
rep (i, n)
if (fcmp((b-a)*(poly[i]-a)) * fcmp((b-a)*(poly[i+1]-a)) <= 0) {
auto vec1 = b-a, vec2 = poly[i+1] - poly[i];
if (fcmp(vec1 * vec2) == 0) {
points.push_back(poly[i]);
points.push_back(poly[i+1]);
} else {
points.push_back(getIntersection(a, b-a, poly[i], poly[i+1]-poly[i]));
}
}
sort(range(points), [] (pt a, pt b) {
return make_pair(a.real(), a.imag()) < make_pair(b.real(), b.imag());
});
points.resize(unique(range(points), [] (pt a, pt b) {
return fcmp(a.real(), b.real()) == 0 && fcmp(a.imag(), b.imag()) == 0;
}) - points.begin());
double cur = 0.0;
assert(points.size() > 1);
for (int i = 1; i < points.size(); i++) {
if (ptOnPoly((points[i]+points[i-1]) / 2.0, poly.data(), n) >= 0) {
cur += abs(points[i] - points[i-1]);
} else {
cur = 0.0;
if (abs(points.back() - points[i]) <= ans) return;
}
ans = max(ans, cur);
}
}

int main() {
cin >> n; poly.resize(n);
rep (i, n) cin >> poly[i];
poly.push_back(poly[0]);
rep (i, n) {
rep (j, i) {
attempt(poly[i], poly[j]);
}
}
printf("%.8f\n", ans);
return 0;
}