#include<bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
vector<ll> a(n), b(n);
for(int i = 0; i < n; i++) {
cin >> a[i];
}
for(int i = 0; i < n; i++) {
cin >> b[i];
}
vector<int> v;
v.push_back(0);
ll ans = 0;
for(int i = 1; i < n; i++) {
while(!v.empty() && b[i] > 0) {
int x = v.back();
if(a[x] > b[i]) {
a[x] -= b[i];
ans += b[i];
b[i] = 0;
} else {
b[i] -= a[x];
ans += a[x];
a[x] = 0;
v.pop_back();
}
}
if(a[i] > 0) v.push_back(i);
}
for(int i = 0; i < n; i++) {
ll x = min(a[i], b[i]);
a[i] -= x;
b[i] -= x;
}
for(int i = 0; i < n; i++) {
ans -= b[i];
}
cout << ans << '\n';
return 0;
}
