#include<stdio.h>
#define N 100000
int x[N+1],y[N+1];
int main(){
    FILE *fin,*fout;
    fin=fopen("aria.in","r");
    fout=fopen("aria.out","w");
    int n,i;
    fscanf(fin,"%d",&n);
    for(i=0;i<n;i++)
        fscanf(fin,"%d%d",&x[i],&y[i]);
    double ans=0;
    x[n]=x[0];
    y[n]=y[0];
    for(i=0;i<n;i++){
        double xt=x[i],yt=y[i],xtn=x[i+1],ytn=y[i+1];
        ans+=(xt*ytn-xtn*yt);
    }
    fprintf(fout,"%.5lf",ans/2);
    return 0;
}