#include<assert.h>
#include<stdio.h>
#include<math.h>
typedef struct P { double x,y; };
P v[100005];
int i,n;
double s;
int main() {
    assert(freopen("aria.in","r",stdin));
    assert(freopen("aria.out","w",stdout));
    assert(scanf("%d",&n)==1),assert(1<=n&&n<=100000);
    for(i=0;i<n;++i) {
        assert(scanf("%lf%lf",&v[i].x,&v[i].y)==2);
        assert(-10000000<=v[i].x&&v[i].x<=10000000);
        assert(-10000000<=v[i].y&&v[i].y<=10000000);
    }
    for(v[n]=v[0],i=0;i<n;++i)
        s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    printf("%lf",fabs(s/2.0));
}