What is wrong:

#include <stdio.h>
#include <string.h>
int i,j,k,flag;
char last;
char a[10001];


int main()
{
 last='.';
 while(gets(a)!=NULL)
 {
  for (i=0;i<strlen(a);i++)
  {
   if (last!='?'&&last!='!'&&last!='.')
    if (a[i]>='A'&&a[i]<='Z')
     a[i]+=32;
    last=a[i];
   }
    for (i=0;i<strlen(a);i++)
    printf("%c",a[i]);
    printf("\n");
  }
    return 0;

 }

HELLO. WORLD
Right answer is:
Hello. World
Your program prints:
Hello. world

Thank you