#include<stdio.h>
#define maxn 100005

FILE *fi,*fo;
int i,n;
double x[maxn],y[maxn];
double a=0;

int main(void){
    fi=fopen("aria.in","r");
    fo=fopen("aria.out","w");
    
    fscanf(fi,"%d",&n);
    for(i=1;i<=n;i++) fscanf(fi,"%lf%lf",&x[i],&y[i]);
    
    x[n+1]=x[1]; y[n+1]=y[1];
    
    for(i=1;i<=n;i++) a+=(1LL*x[i]*y[i+1])-(1LL*x[i+1]*y[i]);
    
    if (a<0) a=-a;
    a/=2;
    fprintf(fo,"%lf",a);
    
    fclose(fi); fclose(fo);
    return 0;
}