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Cod sursa(job #932699)
#include <cstdio>
using namespace std;
int abs(long long x)
{
if (x<0)
return x*(-1);
else
return x;
}
int main()
{
freopen("aria.in","r",stdin);
freopen("aria.out","w",stdout);
long long n,a[100001],b[100001];
scanf("%lld",&n);
long long i,ii,yy;
for (i=1; i<=n; i++)
scanf("%lld%lld",&a[i],&b[i]);
long long s=0;
for (i=1; i<=n; i++)
{
ii=i;
yy=i+1;
if (yy>n)
yy-=n;
s=s+(a[ii]*b[yy]-a[yy]*b[ii]);
}
s=abs(s);
printf("%lld",s/2);
if (s%2==1)
printf(".5");
return 0;
}