#include<stdio.h>
using namespace std;
int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int i,n;
    double s,n1,n2,c1,c2,a,b;
    scanf("%d",&n);
    scanf("%lf%lf",&n1,&n2);
    c1=n1;
    c2=n2;
    s=0;
    for(i=2;i<=n;i++){
        scanf("%lf%lf",&a,&b);
        s=s+(n1*b-a*n2);
        n1=a;
        n2=b;
    }
    s=s+a*c2-c1*b;
    s=s/2;
    printf("%lf",s);
return 0;
}