#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];
ll res = 0;
ll p = N-2;
for(ll u = N-1; u > 0; u--) {
while(p >= u && p > 0) p--;
while(p < u && p >= 0) {
// cout << p << ' ' << u << ' ' << park[p] << ' ' << user[u] << endl;
if (!user[u]) break;
if (!park[p]) {p--; continue;}
ll delta = min(park[p], user[u]);
park[p] -= delta;
user[u] -= delta;
res += delta;
}
}
FOR(i, N) res -= max(0ll, user[i] - park[i]);
cout << res << '\n';
}
