#include <iostream>
using namespace std;
int main() {
int c, r;
int arr[500];
int temp;
int curr = 0;
int result = 0;
cin >> c >> r;
for (int i = 0; i < r; i++)
cin >> arr[i];
temp = arr[0];
for (int i = 0; i < r; i++) {
if (temp <= arr[i]) {
for (int j = curr; j < i; j++)
result += temp - arr[j];
temp = arr[i];
curr = i;
}
}
temp = arr[r-1];
curr = r - 1;
for (int i = r-1; i >= 0 ; i--) {
if (temp < arr[i]) {
for (int j = curr; j > i; j--)
result += temp - arr[j];
temp = arr[i];
curr = i;
}
}
cout << result;
return 0;
}