#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <set>
#include <map>
using namespace std;

int a[110];

int n;

int b[110],c[110];

int main()
{
    cin >> n;

    for(int i = 1;i <= n;i++)
    {
        cin >> a[i];
    }

    for(int i = 1;i <= n;i++)
    {
        cin >> b[i] >> c[i];
    }
    int now = 0;

    int result = 0;
    for(int i = 1;i <= n;i++)
    {
        now += c[i];

        result += now * a[i];

        now -= b[i];
    }

    cout << result << endl;
    return 0;
}