不知为何WA了一个点，感觉没有问题

#include <bits/stdc++.h>
#define pii pair<int,int>
#define mkp(x,y) make_pair(x,y)
using namespace std;
const int maxn=1e4+1;
int n,tot,ans,a[maxn],maxx[maxn],sum[maxn];
bool flag;
pii top[maxn];
int main(){
	scanf("%d",&n);
	for (int i=1;i<=n;i++){
		scanf("%d",&a[i]);
		sum[i]=sum[i-1]+a[i];
	}
	for (int i=1;i<=n;i++){
		if ((a[i]>a[i-1]) ^ flag){
			if (flag){
				while (tot>=2 && maxx[tot-1]>top[tot].second && top[tot].second<a[i-1]) tot--;
				top[++tot]=mkp(i-1,a[i-1]);
				maxx[tot]=max(maxx[tot-1],a[i-1]);
			}
			flag^=1;
		}
	}
//	for (int i=1;i<=tot;i++) printf("%d %d\n",top[i].first,top[i].second);
	for (int i=2;i<=tot;i++) ans+=(top[i].first-top[i-1].first-1)*min(top[i].second,top[i-1].second)-(sum[top[i].first-1]-sum[top[i-1].first]);
	printf("%d\n",ans);
	return 0;
}