#include <iostream>
#include <fstream>
#include <cmath>
#include <iomanip>
#define nmax 100005

using namespace std;

struct point {
    double x, y;
} A[nmax];

int n;
double Aria;

int main()
{
    
    ifstream fi("aria.in");
    ofstream fo("aria.out");
    
    fi >> n;
    for (int i = 1; i <= n; i++)
        fi >> A[i].x >> A[i].y;
    
    A[n+1].x = A[1].x;
    A[n+1].y = A[1].y;
    
    Aria = 0;
    
    for (int i = 1; i <= n; i++)
        Aria = Aria + (A[i].x + A[i+1].x) * (A[i+1].y - A[i].y);
    
    fo << fixed;
    fo << setprecision(5) << Aria/2 << "\n";
    
    fi.close();
    fo.close();
    
    return 0;
}