#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int n;
  cin >> n;
  vector<int> a(n - 1), b(n), c(n);
  rep(i, n - 1) cin >> a[i];
  rep(i, n) cin >> b[i] >> c[i];
  ll ans = 0;
  int now = c[0];
  rep(i, n - 1) {
    ans += a[i] * now;
    now += c[i + 1] - b[i + 1];
  }
  cout << ans << '\n';
  return 0;
}