#include
using namespace std;
int rainWaterHarvesting(int *arr, int size)
{
int *left = new int[size];
int *right = new int[size];
// left most array
int temp = arr[0];
for (int i = 0; i < size; ++i)
{
if (arr[i] > temp )
{
temp = arr[i];
left[i] = temp;
}
else
left[i] = temp;
}
// right most array
int temp1 = arr[size - 1];
for (int i = size - 1; i >= 0; --i)
{
if (arr[i] > temp1)
{
temp1 = arr[i];
right[i] = temp1;
}
else
right[i] = temp1;
}
// Water array
int Water = 0;
for (int i = 0; i < size; ++i)
{
Water += min(left[i],right[i]) - arr[i];
}
return Water;
}
int main(int argc, char const *argv[])
{
int N;
cin >> N;
int *arr = new int[N];
for(int i = 0; i < N; i++)
{
cin >> arr[i];
}
int ans = rainWaterHarvesting(arr,N);
cout<< ans << endl;
return 0;
}