#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

int n;
struct _pct{
    long double x,y;
};
_pct V[100005];

void read(){
    in>>n;
    for(int i=1;i<=n;i++){
        in>>V[i].x>>V[i].y;
    }
}
void solve(){
    long double A=0;
    V[0]=V[n];
    V[n+1]=V[1];
    for(int i=1;i<=n;i++){
        A+=V[i].x*(V[i+1].y-V[i-1].y);
    }
    A/=2;
    out<<setprecision(6)<<fixed<<A;
}
int main(){
    read();
    solve();
    return 0;
}