#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 point pair<double,double>
#define pll pair<ll,ll>
#define all(x) (x).begin(), (x).end()
#define x first
#define y second

using namespace std;

const int nmax = 100005;

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

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    scanf("%d", &n);

    for(i = 1; i <= n; i++)
        scanf("%lf%lf", &p[i].x, &p[i].y);

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

    sol = abs(sol);
    sol/=2.0;

    printf("%.6f\n", sol);

    return 0;
}