#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define FOR(i,N) for(ll i = 0; i < N; i++)
#define all(x) (x).begin(), (x).end()
#define F first
#define S second
signed main() {
cin.tie(0); ios::sync_with_stdio(false);
ll N;
cin >> N;
vector<ll> park(N), user(N);
FOR(i, N) cin >> park[i];
FOR(i, N) cin >> user[i];
vector<ll> bru;
ll res = 0;
for(ll u = N-1; u > 0; u--) {
ll delta = min(park[u-1], user[u]);
park[u-1] -= delta;
user[u] -= delta;
res += delta;
if (!park[u-1] && user[u]) bru.push_back(u);
if (park[u-1] && !user[u]) {
while(bru.size()) {
ll end = bru.back();
delta = min(park[u-1], user[end]);
park[u-1] -= delta;
user[end] -= delta;
res += delta;
if (!user[end]) bru.pop_back();
if (!park[u-1]) break;
}
}
}
FOR(i, N) res -= max(0ll, user[i] - park[i]);
cout << res << '\n';
}
