780 B | 780 chars
#include <bits/stdc++.h>
#define for1(s,n) for(int i = s; i < n; i++)
#define for1j(s,n) for(int j = s; j < n; j++)
#define foreach(k) for(auto i : k)
#define foreachj(k) for(auto j : k)
#define pb(a) push_back(a)
#define sz(a) a.size()
using namespace std;
typedef unsigned long long ull;
typedef long long ll;
typedef vector <int> iv1;
typedef vector <vector<int> > iv2;
typedef vector <ll> llv1;
typedef unsigned int uint;
typedef vector <ull> ullv1;
typedef vector <vector <ull> > ullv2;
ll N, ans, mn;
ll dis[110000];
ll cost[110000];
int main() {
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> N;
for1(1, N) cin >> dis[i];
for1(0, N) cin >> cost[i];
mn = cost[0];
for1(1, N) {
ans += dis[i] * mn;
mn = min(mn, cost[i]);
}
cout << ans;
}