#include <fstream>
#include <iostream>
#include <bits/stdc++.h>

using namespace std;

static inline double arie( double x2, double x1, double y2, double y1 ) {
    return ( x2 - x1 ) / 2 * ( y2 + y1 );
}
double a[100000];
double b[100000];

int main() {
    ios_base::sync_with_stdio( 0 );
    ifstream fin( "arie.in" );
    ofstream fout( "arie.out" );
    int n, i;
    double sum = 0;
    fin >> n;
    for ( i = 0; i < n; i ++ ) {
        fin >> a[i] >> b[i];
    }
    for ( i = 0; i < n - 1; i ++ ) {
        sum += arie( a[i + 1], a[i], b[i + 1],  b[i] );
    }
    sum += arie( a[0], a[n - 1], b[0], b[n - 1] );
    sum = sum > 0 ? sum : -sum;
    fout << setprecision( 6 ) << fixed << sum;
    return 0;
}