#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;

int main(){
    int n;
    cin >> n;
    vector<int> a(n-1);
    rep(i,n-1) cin >> a[i];
    vector<int> b(n),c(n);
    rep(i,n) cin >> b[i] >> c[i];
    int ans = 0;
    int bus = 0;
    rep(i,n-1){
        bus += c[i] - b[i];
        ans += bus * a[i];
    }
    cout << ans << endl;
    return 0;
}