#include <cstdio>
#include <cmath>

#define maxN 100000

using namespace std;

struct points{
    double x, y;
}v[maxN+2];

int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    int N, i;
    double ans;
    scanf("%d", &N);
    for (i=1; i<=N; i++)
        scanf("%lf%lf", &v[i].x, &v[i].y);
    v[N+1]=v[1];
    ans=0;
    for (i=1; i<=N; i++)
        ans+=v[i].x*v[i+1].y-v[i].y*v[i+1].x;
    ans*=0.5;
    printf("%.5lf", fabs(ans));
    return 0;
}