# pragma GCC optimize("Ofast")
# pragma GCC optimization ("unroll-loops")
# include "bits/stdc++.h"
std::pair<int,int> DR[] = {{-1,0},{0,1},{1,0},{0,-1},{-1,1},{-1,-1},{1,1},{1,-1}};
# define ll long long
# define clock (clock() * 1000.0 / CLOCKS_PER_SEC)
# define rc(s) return cout << s,0
# define rcg(s) cout << s;exit(0)
# define _ ios_base::sync_with_stdio(false);cin.tie(0);cerr.tie(0);cout.tie(0);
# define db(x) cerr << #x << " = " << x << '\n'
# define pb push_back
# define mp make_pair
# define all(s) s.begin(),s.end()
# define sz(x) (int)((x).size())
# define int ll
using namespace std;

// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# define LOCAL
# define sim template < class c
# define ris return * this
# define dor > debug & operator <<
# define eni(x) sim > typename \
enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
*this << "[";
for (auto it = d.b; it != d.e; ++it)
*this << ", " + 2 * (it == d.b) << *it;
ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define show(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
int gcd(int a, int b)
{
if(b) return gcd(b,a%b);
return a;
}mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# define ld long double
# define point pair<ld,ld>
# define line pair<point,point>

int n;
ld x[1 << 17],y[1 << 17];
vector<point>vec,lowerpts,upperpts;
ld aria;

ld operator ^(line a,line b)
{
	ld x1 = a.second.first - a.first.first;
	ld y1 = a.second.second - a.first.second;
	ld x2 = b.second.first - b.first.first;
	ld y2 = b.second.second - b.first.second;
	return x1*y2 - y1*x2;
}

int32_t main(){_
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	cin >> n;
	for(int i = 1;i <= n;i++)
	{
		cin >> x[i] >> y[i];
	}
	for(int i = 2;i < n;i++)
	{
		aria += mp(mp(x[1],y[1]),mp(x[i],y[i])) ^ mp(mp(x[1],y[1]),mp(x[i + 1],y[i + 1]));
	}
	rc(fixed << setprecision(6) << abs(aria / 2));
	return 0;
}