#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define f first
#define s second
#define pb push_back
#define yes cout << "yes" << endl;
#define YES cout << "YES" << endl;
#define no cout << "no" << endl;
#define NO cout << "NO" << endl;
void never_give_up() {
int n;
cin >> n;
vector<int>vt(n + 1);
for(int i = 1;i <= n; i++){
cin >> vt[i];
}
int cvb = 0;
for(int i = n; i >= 2; i--){
if(vt[i] >= vt[i - 1]){
int k = vt[i] - vt[i - 1] + 1;
for(int j = i - 1; j >= 1; j--){
vt[j] += k;
}
cvb += k;
//cout << i << " " << k << endl;
}
/*cout << i << " : ";
for(int j = 1;j <= n; j++){
cout << vt[j] << " ";
}
cout << endl;*/
}
cout << cvb << endl;
}
void You_can_do_it() {
//cout << 4096 + 512 + 256 + 128;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int THE_wasd = 1;
//cin >> THE_wasd;
while(THE_wasd--){
never_give_up();
//You_can_do_it();
}
}
//1 2 4 8 16 32 64 128 256 512 1024 2048 4096
