#include<fstream>
#include<cmath>
using namespace std;
int n,x[100001],y[100001],nr;
ifstream f("copaci.in");
ofstream g("copaci.out");
int cmmdc(int a,int b)
{if(b==0)
return a;
int r=a%b;
while(r)
{a=b;
b=r;
r=a%b;
}
return b;
}
int main()
{f>>n>>x[1]>>y[1];
for(int i=2;i<=n;i++)
{f>>x[i]>>y[i];
nr+=x[i-1]*y[i]-x[i]*y[i-1];
nr-=cmmdc(abs(x[i-1]-x[i]),abs(y[i-1]-y[i]))+1;
}
nr+=x[n]*y[1]-x[1]*y[n];
nr-=cmmdc(abs(x[n]-x[1]),abs(y[n]-y[1]))+1;
g<<(nr+n)/2+1;
return 0;
}