#include <bits/stdc++.h>
using namespace std;
#define int long long
#define vi vector<int>
#define ff first
#define ss second
#define pb push_back
#define all(a) (a).begin(),(a).end()
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
const int mod = 1e9 + 9;
const int mx = 3e5 + 5;
int ans = 0;
signed main() {
int n; cin >> n;
vector<int> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
vector<int> b(n);
for (int i = 0; i < n; i++) {
cin >> b[i];
}
set<int> st;
for (int i = 0; i < n; i++) {
st.insert(i);
}
for (int i = n - 1; i >= 0; i--) {
vi v;
auto x = st.upper_bound(i);
while (x != st.end()) {
int xx = *x;
int mn = min(a[i], b[*x]);
a[i] -= mn;
b[*x] -= mn;
ans += mn;
x++;
if (a[i] == 0) break;
v.pb(*x);
}
rep(i, v.size()){
if (b[v[i]] == 0) {
st.erase(v[i]);
}
}
}
for (int i = 0; i < n; i++ ) {
int t = min(a[i], b[i]);
a[i] -= t; b[i] -= t;
}
for (int i = 0; i < n; i++) {
if (b[i] > 0) {
ans -= b[i];
}
}
cout << ans << "\n";
return 0;
}
