#include <bits/stdc++.h>
using namespace std;
#define mod 9973
int a[1000009];
int n,r,s,cand;
int v[999999];
long long x,nrd;
void cit()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
}
int main()
{
   cit();
    scanf("%d",&n);
    for(int i=1;i<=n;i++)

        scanf("%d %d",&a[i],&v[i]);
    for(int i=1;i<=n;i++)
        s+=(a[i]*v[i+1]-a[i+1]*v[i])/2;

    s+=(a[n]*v[1]-a[1]*v[n])/2;
    printf("%d\n",s);
    return 0;

}