#include<cstdio>
#define M 1<<16
char r[M];
int e=M,n,i;
double x,y,a,b,c,d;
long double s;
inline char C()
{
    if(e==M)
        fread(r,1,M,stdin),e=0;
    return r[e++];
}
inline double D()
{
    char c=C(),s=43;
    double a=0;
    for(;c<48;c=C())
        s=c;
    for(;c>47;c=C())
        a=a*10+c-48;
    if(c==46)
    {
        double b=0,p=1;
        for(;(c=C())>47;p*=10)
            b=b*10+c-48;
        a+=b/p;
    }
    return s==45?-a:a;
}

int main()
{
    freopen("aria.in","r",stdin),freopen("aria.out","w",stdout);
    n=D(),c=D(),d=D(),a=c,b=d;
    for(i=1;i<n;i++)
        x=D(),y=D(),s+=(long double)a*y-(long double)x*b,a=x,b=y;
    s+=(long double)x*d-(long double)y*c;
    /*read(n,x1,y1);
    xi=x1; yi=y1;
    for(i=2; i<=n; i++)
    {
        read(xj,yj);
        s = s + xi*yj-xj*yi;
        xi = xj; yi = yj;
    }
    s = s + xj*y1-x1*yj;
    for(n=D(),i=0;i<n;i++)
        x[i]=D(),y[i]=D();
    x[n]=x[0],y[n]=y[0];
    for(i=0;i<n;i++)
        s+=(long double)x[i]*y[i+1]-(long double)x[i+1]*y[i];*/
    printf("%.5Lf",s/2);
}