// Infoarena. Arhiva Educationala. Aria.
#include<iostream>
#include<fstream>
#include<cmath>
using namespace std;

int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    cout.precision(12);

    int N;
    double X[100003], Y[100003];

    cin>>N;
    for(int i=1;i<=N;i++){
        cin>>X[i]>>Y[i];
    }

    double S=0;
    for(int i=1;i<N;i++){
        S+=X[i]*Y[i+1]-X[i+1]*Y[i];
    }

    S+=X[N]*Y[1]-X[1]*Y[N];

    cout<<fabs(S/2);
}