#include<iostream>
#include<vector>
using namespace std;
#define int long long
signed main() {
int N;
cin>>N;
vector<int> slot(N+1), person(N+1);
for(int i=0; i<N; i++) cin>>slot[i];
for(int i=0; i<N; i++) cin>>person[i];
int ptr = 0, ans = 0;
for(int i=0; i<N; i++) {
int need = person[i];
for(int t = ptr; t<i && need; t++) {
int take = min(need, slot[t]);
slot[t] -= take;
need -= take;
ans += take;
}
while(ptr<N && slot[ptr] == 0) ptr++;
person[i] = need;
}
int cnt = 0;
for(int i=0; i<N; i++) {
int take = min(slot[i], person[i]);
slot[i] -= take;
person[i] -= take;
}
for(int i=0; i<N; i++) {
ans -= person[i];
}
cout<<ans;
}
