#include <stdio.h>

int main(){
    int n;
    scanf("%d",&n);
    int r[n], i, br=0;
    for(i=0;i<n;i++) scanf("%d",&r[i]);
    for(i=n-1;i>0;i--){
                        if(r[i]<=r[i-1]){
                                         br=br+r[i-1]-r[i]+1;
                                         r[i-1]=r[i]-1;
                                         }
                        }
    printf("%d",br);
    }