#include <cstdio>
#define Nmax 100005

using namespace std;

struct pc
{
    double x,y;
};
pc a[Nmax];

inline double ArieTr(pc A, pc B, pc C)
{
    return 0.5*(A.x*B.y+B.x*C.y+C.x*A.y-C.x*B.y-A.x*C.y-A.y*B.x);
}

int main()
{
    int i,N;
    double Aria=0;
    pc P;
    freopen ("aria.in","r",stdin);
    freopen ("aria.out","w",stdout);
    scanf("%d", &N);
    for(i=1;i<=N;++i)
        scanf("%lf%lf", &a[i].x,&a[i].y);
    P.x=0.009; P.y=100.578;
    for(i=1;i<N;++i)
        Aria+=ArieTr(a[i],a[i+1],P);
    Aria+=ArieTr(a[N],a[1],P);
    printf("%lf\n", Aria);
    return 0;
}