#include<cstdio>
#include<fstream>
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<vector>
#include<bitset>
#include<deque>
#include<queue>
#include<set>
#include<map>
#include<cmath>
#include<cstring>
#include<ctime>
#include<cstdlib>
#include<unordered_map>

#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int,int>
#define pll pair<ll,ll>

#define point pair<double,double>
#define x first
#define y second

using namespace std;

const int nmax = 100005;

int n, i;
double a;
point p[nmax];

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");

    fin >> n;
    for(i = 1; i <= n; i++)
        fin >> p[i].x >> p[i].y;

    p[n + 1] = p[1];

    for(i = 1; i <= n; i++)
        a += p[i].x * p[i + 1].y - p[i].y * p[i + 1].x;

    if(a < 0) a = -a;
    a /= 2.0;

    fout << fixed << setprecision(6) << a;

    return 0;
}