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

using namespace std;

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

int n;
struct pct{
    long double x,y;
} P[100005];

void read(){
    in>>n;
    for(int i=1;i<=n;i++){
        in>>P[i].x>>P[i].y;
    }
}
void solve(){
    long double S=0;
    P[0]=P[n];
    P[n+1]=P[1];
    for(int i=1;i<=n;i++){
        S+=P[i].x*(P[i+1].y-P[i-1].y);
    }
    S/=2.0;
    out<<setprecision(5)<<fixed<<fabs(S);
}
int main(){
    read();
    solve();
    return 0;
}