#define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#include <iostream>
#include <map>
#include <set>
#include <list>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <string>
#include <vector>
#include <algorithm>
#include <utility>
#include <queue>
#include <iomanip>
#include <functional>

#define SIZE 1005
#define INF 100000000
#define ll long long
#define FD(d) fixed << setprecision(d)
#define MOD 1000000007
     
using namespace std;
 
#define ll long long
 
typedef pair<int, int> Pa;

int main()
{
	ll n;

	cin >> n;

	ll res = 0;
	for (ll i = 0; i < n; i++) {
		ll c, d;
		cin >> c >> d;
		res += ((c + 1) / 2) * d;
	}
	cout << res << endl;

	return (0);
}