#include <iostream>
using namespace std;


int n; int v[1001];

int main() {

    cin >> n;
    
    int i; int r=0;
    for(i=0; i<n; i++)
        cin >> v[i];
      
      
    for(i=n-1; i>0; i--)
        if(v[i-1] >= v[i]) {
            r+= v[i-1] - v[i] + 1;
            v[i-1] = v[i]-1;
        }
        
    cout << r << endl;
        

    //system("pause");
    }