#include <bits/stdc++.h>
#define int long long
using namespace std;
const int maxn = 3e5 + 6;
int st[4*maxn+5], lazy[4*maxn+5];
void build(int id, int l, int r, vector<int> &a){
lazy[id] = -1;
if (l == r){
st[id] = a[l];
return;
}
int mid = (l + r) >> 1;
build(id << 1, l, mid, a);
build(id << 1 | 1, mid+1, r, a);
st[id] = st[id << 1] + st[id << 1 | 1];
}
void down(int id, int l, int r){
int mid = (l + r)/2;
if (lazy[id] != -1){
lazy[id << 1] = lazy[id];
lazy[id << 1 | 1] = lazy[id];
// set all into lazy[id]
st[id << 1] = lazy[id]*(mid - l + 1);
st[id << 1 | 1] = lazy[id]*(r - mid);
lazy[id] = -1;
}
}
int get(int id, int l, int r, int ul, int ur){
if (l > ur || r < ul) return 0;
if (ul <= l && r <= ur) return st[id];
down(id, l, r);
int mid = (l + r) >> 1;
return get(id << 1, l, mid, ul, ur) + get(id << 1|1, mid+1, r, ul, ur);
}
void update(int id, int l, int r, int ul, int ur, int val){
if (l > ur || r < ul) return;
if (ul <= l && r <= ur){
st[id] = val * (r - l + 1);
lazy[id] = val;
return;
}
down(id, l, r);
int mid = (l + r) >> 1;
update(id << 1, l, mid, ul, ur, val);
update(id << 1 | 1, mid+1, r, ul, ur, val);
st[id] = st[id << 1] + st[id << 1 | 1];
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(nullptr);
int n;
cin >> n;
vector<int> x(n+1, 0), y(n+1), pref(n+1, 0);
for (int i = 1; i <= n; i++){
cin >> x[i];
pref[i] = x[i] + pref[i-1];
}
for (int i = 1; i <= n; i++){
cin >> y[i];
}
int ans = 0;
stack<pair<int, int>> st;
for (int i = 2; i <= n; i++){
st.push({i-1, x[i-1]});
while(!st.empty() && y[i] > 0){
pair<int, int> pa = st.top();
st.pop();
int num = pa.second;
if (num > y[i]){
st.push({pa.first, num - y[i]});
ans += y[i];
x[pa.first] = num - y[i];
y[i] = 0;
}
else{
y[i] -= num;
ans += num;
x[pa.first] = 0;
}
}
}
for (int i = 1; i <= n; i++){
ans -= y[i] - min(x[i], y[i]);
}
cout << ans;
}
