#pragma GCC optimize ("O3")
#include <bits/stdc++.h>
using namespace std;
#define x first
#define y second
#define dbg(x) cout << #x << '=' << x << '\n';
#define ll long long
#define pi pair<int,int>
#define pl pair<ll,ll>
#define pd pair<double,double>
#define ld long double
#define pld pair<ld,ld>
#define lg length()
#define sz size()
#define pb push_back
#define MAXN 100005
#define INF 1000000005
#define LINF 1000000000000000005
#define x1 xdddddddddddddddddd
#define y1 ydddddddddddddddddd

int n;

double ans,x[100005],y[100005];

int32_t main(){
    ios_base :: sync_with_stdio(0); cin.tie(); cout.tie();
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    cin >> n;
    for(int i=0;i<n;i++) cin >> x[i] >> y[i];
    for(int i=0;i<n;i++) ans+=(x[i]*y[(i+1)%n]-x[(i+1)%n]*y[i]);
    ans/=2;
    cout << abs(ans);
}