#include <fstream>
#include <iomanip>
#define nmax 100100
using namespace std;

struct punct{int x,y;}Punct[nmax];
int N;
double A;

void solve() {

    Punct[N+1]=Punct[1];

    for(int i=1;i<=N;i++)
        A+=Punct[i].x*Punct[i+1].y-Punct[i+1].x*Punct[i].y;

    A/=2;

}
void read() {

    ifstream in("aria.in");

    in>>N;
    for(int i=1;i<=N;i++)
        in>>Punct[i].x>>Punct[i].y;

    in.close();

}
void write() {

    ofstream out("aria.out");
    out<<fixed<<setprecision(4)<<A<<'\n';
    out.close();

}
int main() {

    read();
    solve();
    write();

    return 0;

}