#include <bits/stdc++.h>
#define pb push_back
#define SS ios_base::sync_with_stdio(0);cin.tie(nullptr);cout.tie(nullptr);
#define int long long
#define all(v) v.begin(),v.end()
using namespace std;
const int N = 1e4, inf = 1e9 + 1, mod = 998244353;
void solve(){
int n;
cin >> n;
int x[n + 1], y[n + 1];
for(int i = 1; i <= n; i++){
cin >> x[i];
}
for(int i = 1; i <= n; i++){
cin >> y[i];
}
reverse(x + 1, x + n + 1);
reverse(y + 1, y + n + 1);
int j = 1;
int ans = 0;
for(int i = 1; i <= n; i++){
while(j <= i) j++;
while(j <= n && y[i] > 0){
if(y[i] >= x[j]){
y[i] -= x[j];
ans += x[j];
x[j] = 0;
}
else{
x[j] -= y[i];
ans += y[i];
y[i] = 0;
}
j++;
}
if(y[i] > 0){
y[i] -= x[i];
y[i] = max(0ll, y[i]);
ans -= y[i];
}
}
cout << ans;
}
signed main(){
SS
// freopen("trains.in", "r", stdin);
// freopen("trains.out", "w", stdout);
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
