#include <fstream>
#include <iomanip>
#define Nmax 100005

using namespace std;

int n;
long double ARIA_BALTI_E_BOSSUL_MEU;

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

inline long double Aria(pc A, pc B, pc C)
{
    return (B.y*A.x+B.x*C.y+C.x*A.y-C.x*B.y-A.x*C.y-B.x*A.y)/2;
}

















int main()
{
    int i;
    ifstream fin ("aria.in");
    ofstream fout ("aria.out");
    fin>>n;
    for(i=1;i<=n;++i)
        fin>>a[i].x>>a[i].y;
    pc PUNCT_RANDOM_MUIE_STEAUA;
    PUNCT_RANDOM_MUIE_STEAUA.x=0.4;
    PUNCT_RANDOM_MUIE_STEAUA.y=69;
    for(i=1;i<n;++i)
        ARIA_BALTI_E_BOSSUL_MEU+=Aria(PUNCT_RANDOM_MUIE_STEAUA,a[i],a[i+1]);
    ARIA_BALTI_E_BOSSUL_MEU+=Aria(PUNCT_RANDOM_MUIE_STEAUA,a[n],a[1]);
    fout<<setprecision(10)<<fixed;
    fout<<ARIA_BALTI_E_BOSSUL_MEU;
    return 0;
}