#include<stdio.h>
#include<math.h>
struct P { double x,y; }v[100000];
int i,n;
long double s;
int main() {
    freopen("aria.in","r",stdin),freopen("aria.out","w",stdout),scanf("%d",&n);
    for(i=0;i<n;++i)
        scanf("%lf%lf",&v[i].x,&v[i].y);
    for(v[n]=v[0],i=0;i<n;++i)
        s+=(long double)(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    printf("%llf",fabs(s/2.0));
}