#include <iostream>
#include <fstream>

using namespace std;

int main()
{ int n,a,b,a1,b1;
 float s=0;

    ifstream f("arie.in");
    ofstream g("arie.out");
    f>>n;
    f>>a>>b;
    n--;
    while(n)
    {
        f>>a1>>b1;
        s+=((b1+b)*(a1-a))/2;
        a=a1;
        b=b1;
        n--;
    }
    if(s<0) s*=(-1);
    g<<s;

    return 0;
    }