【題目敘述】https://atcoder.jp/contests/abc176/tasks/abc176_c
#include <iostream>
using namespace std;
int n, a[200005];
long long ans;
int main() {
cin >> n;
for (int i = 1; i <= n; i++){
cin >> a[i];
if (a[i] < a[i-1]){
ans += a[i-1]-a[i];
a[i] = a[i-1];
}
}
cout << ans << "\n";
}