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

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

    cout.precision(20);

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

    InFile>>N;
    for(int i=0;i<N;i++){
        InFile>>X[i]>>Y[i];
    }
    X[N]=X[0]; Y[N]=Y[0];

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

    cout<<fabs(S/2.0);
}