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#include <iostream>
#include <bits/stdc++.h>
//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")

#define mp make_pair
#define pb push_back
#define eb emplace_back
#define x first
#define y second
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define RFOR(i,b,a) for (int i = (b) - 1; i >= (a); i--)

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef vector<int> veci;
typedef vector<bool> vecb;
typedef vector<vector<int>> vvi;
typedef vector<vector<bool>> vvb;

const int INF_I = 1e9;
const ll INF_LL = 1e18;
const int MOD = 1000000007;
const double eps = 1e-6;

int main() {

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);

    int n;
    cin >> n;
    vector<pl> a(n);
    for (int i = 0; i < n; i++)
        cin >> a[i].x >> a[i].y;
    ld S = 0;
    for (int i = 0; i < n; i++)
        S += ld(a[i].x * a[(i+1)%n].y - a[i].y * a[(i+1)%n].x) / 2;
    cout.precision(3);
    cout << fixed << fabs(S) << endl;

    return 0;
}