#include<cstdio>
#include<cmath>
struct pct{
    double x;
    double y;
}v[100100];
int n,i;
double ss,s;
FILE *f,*g;
double mod(int a){
    if(a<0)
        return -a;
    return a;
}
int main(){
    f=fopen("aria.in","r");
    g=fopen("aria.out","w");
    fscanf(f,"%d",&n);
    for(i=1;i<=n;i++){
        fscanf(f,"%lf%lf",&v[i].x,&v[i].y);
    }
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(i=1;i<=n;i++){
        s = ( v[i].x * v[i + 1].y - v[i].y * v[i + 1].x) / 2;
        ss+=s;
    }
    fprintf(g,"%.5lf",ss);


    fclose(f);
    fclose(g);
    return 0;
}