#include<bits/stdc++.h>
using namespace std;
const int MAXN=3e5+5;
int n,a[MAXN],b[MAXN],s[MAXN];
signed main() {
	scanf("%d",&n);
	for(int i=1;i<=n;++i) scanf("%d",&a[i]);
	for(int i=1;i<=n;++i) scanf("%d",&b[i]);
	int w=0;
	for(int i=1,j=0;i<=n;++i) {
		while(b[i]&&j) {
			int x=min(b[i],a[s[j]]);
			w+=x,b[i]-=x,a[s[j]]-=x;
			if(!a[s[j]]) --j;
		}
		s[++j]=i;
	}
	for(int i=1;i<=n;++i) w-=max(0,b[i]-a[i]);
	printf("%d\n",w);
	return 0;
}