//
//  aria.cpp
//  probleme
//
//  Created by Mihnea Buzoiu on 6/5/21.
//

#include <stdio.h>
#include <iostream>
#include <cmath>

using namespace std;

const int N = 1e5 + 2;

long double x[N];
long double y[N];

int main(int argc, const char * argv[]) {
    
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    
    int n;
    scanf("%d", &n);
    
    for (int i=0; i<n; i++)
        scanf("%Lf %Lf", &x[i], &y[i]);
    
    x[n] = x[0];
    y[n] = y[0];
    
    long double arie = 0;
    for (int i=0; i<n; i++)
        arie += (x[i] * y[i+1] - x[i+1] * y[i]);
    
    printf("%.8Lf", (abs(arie) / 2.0));
}

/*
4
-2 -2
2 -2
2 2
-2 2
*/