#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
using namespace std;
using namespace __gnu_pbds;
#define g0(x) get<0>(x)
#define g1(x) get<1>(x)
#define g2(x) get<2>(x)
#define mt make_tuple
#define mp make_pair
#define pb push_back
#define int long long
#define f first
#define s second
#define pll pair<long long, long long>
#define iii tuple<int,int,int>
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
signed main(){
int n;cin>>n;
vector<int> x(n+1, 0), y(n+1, 0);
for(int i=1;i<=n;i++){
cin>>x[i];
}
int tot=0;
for(int i=1;i<=n;i++){
cin>>y[i];
tot += y[i];
}
int ans=0, ptr=0, missed=0;
for(int i=1;i<=n;i++){
ptr=max(ptr, i+1);
while(ptr <= n and x[i]){
int u=min(x[i], y[ptr]);
y[ptr] -= u;
x[i] -= u;
ans += u;
if(y[ptr] == 0)ptr++;
}
int s=min(x[i], y[i]);
x[i] -= s;
y[i] -= s;
int d=min(missed, x[i]);
ans -= d;
missed -= d;
missed += y[i];
}
cout<<ans;
}
