//
// Created by liasa on 14/05/2026.
//
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define v vector
#define lp(i, s, e) for (ll i = s; i < e; ++i)
#define pll pair<ll, ll>
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
ll n;
cin >> n;
v<ll> x(n), y(n);
lp(i, 0, n) cin >> x[i];
lp(i, 0, n) cin >> y[i];
ll cur_ans = 0;
ll lf = 0, change = 0;
ll minus = 0;
lp(i, 0, n) {
ll rooms = x[i];
ll pep = y[i];
ll pls = min(change, pep);
cur_ans += pls;
pep -= pls;
change -= pls;
minus += pls;
ll plus = min(pep, lf);
cur_ans += plus;
pep -= plus;
lf -= plus;
ll ass_to_room_i = min(pep, rooms);
pep -= ass_to_room_i;
rooms -= ass_to_room_i;
lf += rooms;
change += ass_to_room_i;
cur_ans -= pep;
}
cur_ans -= minus;
cout << cur_ans;
}
