#include<iomanip>
#include<iostream>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
using namespace std;
int main()
{
	int n,s,k,ji=1,he=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>s>>k;
		ji*=s;
		he+=k;
	}
	cout<<abs(he-ji);
	return 0;
}