保证AC

#include <bits/stdc++.h>
using namespace std;
long long a[200010];
int main(){
    long long n, sum = 0; 
    cin>>n;
    for (int i = 1; i <= n; i++){
    	cin >> a[i];
    }
    for (int i = 1; i < n; i++){
    	if (a[i] > a[i + 1]){
            sum += a[i] - a[i + 1];
            a[i + 1] += a[i] - a[i + 1];
        }
    }
    cout << sum;
}