#include<stdio.h>
int main()
{
    int n,i,d[100],k=0,zbroj=0,zbroj1=0;
    scanf("%d",&n);
    for(i=0;i<=n-1;i++){scanf("%d",&d[i]);
  zbroj=zbroj+d[i];}
  
    for(i=n-1;i>=0;i--){printf("n=%d ",d[i]);
    
    if(d[i-1]>=d[i]) d[i-1]=d[i-1]-(d[i-1]-d[i]+1);
    zbroj1=zbroj1+d[i];
     
    }
    k=zbroj-zbroj1;
 printf("%d ",k);

return 0;
}