//
//  aria.cpp
//  probleme
//
//  Created by Mihnea Buzoiu on 6/3/21.
//

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

using namespace std;

double x[100002];
double y[100002];

int main(int argc, const char * argv[]) {
    
    FILE * fin = fopen("aria.in", "r");
    FILE * fout = fopen("aria.out", "w");
    
    int n;
    fscanf(fin, "%d", &n);
    
    for (int i=0; i<n; i++)
        fscanf(fin, "%lf %lf", &x[i], &y[i]);
    
    x[n] = x[0];
    y[n] = y[0];
    
    double arie = 0;
    for (int i=0; i<n; i++)
        arie += (x[i] * y[i+1] - x[i+1] * y[i]);
    
    fprintf(fout, "%lf", (arie / 2.0));
}

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