/*
    Keep It Simple!
*/

#include<fstream>
#include<iomanip>
using namespace std;

#define MaxN 100005

struct point{ double x,y; } P[MaxN];
double aria;
int N;


int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");

    f >> N;

    for(int i=1;i<=N;i++)
        f >> P[i].x >> P[i].y;
    P[0] = P[N];

    for(int i=0;i<N;i++)
        aria += (P[i].x*P[i+1].y)-(P[i+1].x*P[i].y);

    if(aria < 0) aria = -aria;
    aria /= 2;

    g << setprecision(10) << aria;
}