#include <cstdio>
#include <iostream>
#include <cstdlib>
using namespace std;
int n, b[100005], sol, raz;
int main (){
    scanf("%d", &n);
    
for(int i = 0;i < n; i++)
    scanf("%d", &b[i]);
    
for(int i = n - 1; i > 0; i--){
   if(b[i] <= b[i-1]){
           raz = abs(b[i] - b[i-1]);
           sol += raz + 1;
           b[i-1] = b[i-1] - raz - 1;
           }
}   

printf("%d\n", sol);
    
    //system("PAUSE");
 return 0;   
}