#include <bits/stdc++.h>
using namespace std;


int main(void){
long long a,b,c,sum=0;
cin>>a;
for (int i = 0; i < a; ++i) {
	cin>>b>>c;
	if(b%2==0){
		sum=sum+((b/2*c));
	}else{
		sum=sum+(((b+1)/2*c));
	}
}
cout<<sum<<endl;

}