#include <cstdio>

using namespace std;

#define NMAX 100005

struct Point
{
    double x,y;
};

Point V[NMAX];

int N;

double A;

void Read()
{
    freopen("aria.in","r",stdin);
    scanf("%d",&N);

    for(int i=1;i<=N;i++)
        scanf("%lf%lf",&V[i].x,&V[i].y);

    V[N+1] = V[1];
}

void Solve()
{

    for(int i=1;i<N+1;i++)
        A += V[i].x * V[i+1].y - V[i+1].x * V[i].y;
    A /= 2;
}

void Print()
{
    freopen("aria.out","w",stdout);
    printf("%.6lf",A);
}

int main()
{
    Read();
    Solve();
    Print();
    return 0;
}