#include <bits/stdc++.h>
#define PB push_back
#define all(x) x.begin(),x.end()
#define sz(x) ((int)x.size())
#define i2 array<int,2>
#define l2 array<ll,2>
using namespace std;
typedef long double ld;
typedef long long ll;
const int N = 100100;
const ld E = 1e-9;
vector<i2> pts;
bool mrk[N];
int x[N], y[N], n, nt[N], pr[N];
ld st[4 * N], psh[4 * N], cans[N];
ld sqr(ld x){ return x * x;}
ld dist(i2 a, i2 b){
return sqrt(sqr(a[0] - b[0]) + sqr(a[1] - b[1]));
}
int next(int i) { return (i + 1) % sz(pts); }
ll area(i2 a, i2 b, i2 c){
return (ll)(a[0] - b[0]) * (a[1] - c[1]) - (ll)(a[1] - b[1]) * (a[0] - c[0]);
}
bool ok(i2 a, i2 b, i2 c, i2 d){
i2 cand = d;
cand[0] += -c[0] + b[0];
cand[1] += -c[1] + b[1];
return area(a, b, cand) > 0;
}
void ans(){
pts.clear();
for (int i = 0; i < n; i++)
if (!mrk[i])
pts.PB({x[i], y[i]});
ld ans = 0, sum = dist(pts[0], pts[1]);
for (int i = 0, j = 1; i < sz(pts); i++){
if (i > 0)
sum -= dist(pts[i], pts[i - 1]);
while (ok(pts[i], pts[next(i)], pts[j], pts[next(j)])) {
sum += dist(pts[j], pts[next(j)]);
j = next(j);
}
cans[i] = sum;
}
}
void build(int v, int l, int r){
if (l == r){
st[v] = cans[l];
return;
}
int md = (l + r) >> 1;
build(v + v, l, md);
build(v + v + 1, md + 1, r);
st[v] = max(st[v + v], st[v + v + 1]);
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef _LOCAL
freopen("in.txt","r",stdin);
#endif // _LOCAL
cin >> n;
for (int i = 0; i < n; i++) {
cin >> x[i] >> y[i];
nt[i] = (i + 1) % n;
pr[i] = (i - 1 + n) % n;
}
ans();
build(1, 0, n - 1);
cout << fixed << setprecision(10) << st[1];
// int qq; cin >> qq;
//
// for (; qq; qq--){
// int x; cin >> x; x--;
// mrk[x] = 1;
//
// ans();
// }
return 0;
}
