#include <stdio.h>
int main(){
  int N;
  scanf("%d",&N);
  int A[N];
  for (int i = 0; i < N ; i++) scanf("%d",&A[i]);
  
  int flag = 1;
  int ans = 0;
  while (flag){
    flag = 0;
    for (int i = 0;i < N ; i++){
      for (int j = i+1 ; j < N ; j++){
        if (A[i]-A[j]==1){
          int k = A[j]; ans++;
          A[j] = A[i] ; A[i] = k; flag = 1;
        }
      }
    }
  }
  printf ("%d\n",ans);
}

/*高速誤解法テスト*/