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#include <bits/stdc++.h>
using namespace std;
struct punct
{
double x, y;
}a[100010];
int n;
long double aria;
int main()
{
FILE *f=fopen("aria.in","r");
fscanf(f,"%d",&n);
for(int i=1; i<=n; i++)
fscanf(f,"%lf %lf", &a[i].x, &a[i].y);
for(int i=1; i<n; i++)
aria+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
aria+=a[n].x*a[1].y-a[1].x*a[n].y;
if(aria<0)aria=-aria;
aria/=2;
FILE *g=fopen("aria.out","w");
fprintf(g,"%lf",aria);
return 0;
}