#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;
const int Nmax = 100005;

double X[Nmax], Y[Nmax];

inline double area(int a, int b)
{
    return X[a]*Y[b] - Y[a]*X[b];
}

int main()
{
    ifstream f ("aria.in");
    ofstream g ("aria.out");
    
    int N;
    double result = 0;
    f >> N;
    for (int i = 0; i < N; i++)
        f >> X[i] >> Y[i];
    for (int i = 0; i < N-1; i++)
        result += area(i, i+1);
    
    result += area(N-1, 0);
    result /= 2;
    if (result < 0)
        result = -result;
    g << fixed << result << '\n';

    return 0;
}