int a[100001][3];
#include<stdio.h>
using namespace std;
int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int i,n;
    double s;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
        scanf("%d%d",&a[i][1],&a[i][2]);
    s=0;
    for(i=1;i<=n;i++){
        if(i<n)
            s=s+a[i][1]*a[i+1][2]-a[i+1][1]*a[i][2];
        else
            s=s+a[i][1]*a[1][2]-a[1][1]*a[i][2];
    }
    s=s/2;
    printf("%f",s);
return 0;
}