/* Solution to practice problem "tower" by Bob Roos */
#include <stdio.h>

int main() {
  int order[100];
  int i,n,left;

  while (scanf("%d",&n) && n > 0) {
    left = n;
    for (i = 0; i < n; i++) {
      scanf("%d",&order[i]);
      if (order[i] >= left)
        left--;
      else
        left = order[i] - 1;
    }
    if (left >= 0)
       printf("0\n");
    else
       printf("%d\n",-left);
  }
  return 0;
}