#include <iostream>

using namespace std;
int n;
int main(void) {
   cin >> n;
   int a[n];
   for(int i=1; i<=n; i++) cin >> a[i];
   int rez=0;
   for(int i=n; i>0; i--) {
           if(a[i]<=a[i-1]) {rez+=(a[i-1]-a[i])+1; a[i-1]-=(a[i-1]-a[i])+1;}
   }
   cout << rez << endl;
   return 0;
   }