#include<cstdio>
#define M 1<<16
char r[M];
int e=M;
double a,b,c,d,x,y;
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='+';
    double a=0;
    for(;c<48||c>57;c=C())
        s=c;
    for(;c>47&&c<58;c=C())
        a=a*10+c-'0';
    if(c=='.')
    {
        double b=0,p=1;
        for(;(c=C())>47&&c<58;p*=10)
            b=b*10+c-'0';
        a+=b/p;
    }
    return s=='-'?-a:a;
}

int main()
{
    freopen("aria.in","r",stdin),freopen("aria.out","w",stdout);
    n=D(),x=D(),y=D(),a=c=x,b=d=y,n--;
    while(n--)
        x=D(),y=D(),s+=(long double)a*y-(long double)b*x;
    s+=(long double)a*d-(long double)b*c,
    printf("%.5Lf",s/2);
}