#include <bits/stdc++.h>
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
using namespace std;
#define int long long
#define bochi_orz cin.tie(0);cin.sync_with_stdio(0);
signed main() {
bochi_orz
int n;
cin >> n;
vector<int> x(n), y(n);
for (int i = 0; i < n; i++){
cin >> x[i];
}
for (int i = 0; i < n; i++){
cin >> y[i];
}
int sum = 0;
int net = 0;
vector<int> vec;
for (int i = 0; i < n; i++){
if (vec.size() == 0){
sum += x[i];
if (x[i] != 0){
vec.push_back(i);
}
continue;
}
while (y[i] > 0 && sum > 0){
int dec = min(y[i], x[vec.back()]);
x[vec.back()] -= dec;
sum -= dec;
y[i] -= dec;
net += dec;
if (x[vec.back()] == 0){
vec.pop_back();
}
}
sum += x[i];
if (x[i] != 0){
vec.push_back(i);
}
}
for (int i = 0; i < n; i++){
y[i] -= x[i];
net -= max(y[i], (int)0);
}
cout << net << "\n";
}
/*
____ ___ ___ ____ _____ ____ ____ ___
| | | | \ | | /| | | | \
| | | | | |____ | | | _ |____ |___/
| | | | | | | | | | | | \
|____ |___| |___/ |____ | __|__ |____| |____ | \
*/
