#include <bits/stdc++.h>
#define ll long long
using namespace std;
int N;
long double rs;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct{
   double x, y;  
};
punct A[100005];
int main(){
    ios_base::sync_with_stdio(0);
    fin.tie(0);
    fin >> N;
    for(int i = 1; i <= N; i++)
        fin >> A[i].x >> A[i].y;
    A[0] = A[N];
    A[N+1] = A[1];
    for(int i = 1; i <= N; i++)
        rs += A[i].x*(A[i+1].y - A[i-1].y);
    fout << setprecision(6) << fixed << abs(rs/2.0);
    return 0;
}