#include <iostream>
using namespace std;
int a[300005], b[300005], pre[300005];
int main(){
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
int n, Ans = 0;
cin>>n;
for (int i=1;i<=n;i++)
cin>>a[i];
for (int i=1;i<=n;i++)
cin>>b[i], pre[i] = pre[i-1] + b[i];
for (int i=1, ptr = 1;i<=n;){
if (ptr < i){
int k = min(a[ptr], b[i]);
a[ptr] -= k, b[i] -= k;
ptr += !a[ptr], i += !b[i];
Ans += k;
}
else if (ptr == i){
int k = a[ptr] - min(pre[n] - pre[i], a[ptr]);
a[ptr] -= min(b[i], k);
Ans -= max(0, b[i] - k), i++;
}
else
Ans -= b[i], i++;
}
cout<<Ans<<'\n';
}
