Count Sub String From Given String – C Program

#include<stdio.h>
#include<string.h>

int main()
{

 int i,j,count=0;
 char firststr[100],substr[10];

 printf("Please, Enter the main string = ");
 gets(firststr);

 printf("\nPlease, Enter the sub string = ");
 gets(substr);

 for(i=0; firststr[i]!='\0'; i++)
 {
  if(firststr[i]==substr[0])
   {
     for(j=0; firststr[i]==substr[j] && substr[j]!='\0'; i++,j++);
     if(substr[j]=='\0')
      count++;
   }
 }

 printf("The String count = %d",count);

 return 0;
}

Please, Enter the main string = This is a my pen.
Please, Enter the sub string = is

The String count = 2