C program to print UPPERCASE and LOWERCASE letters in a string

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

int main()
{

    int i,upper=0,lower=0;
    char str[100];
    printf("Enter the String:\n");
    gets(str);
    for(i=0; str[i]!=0; i++)
    {
        if(str[i]>='A' && str[i]<='Z')
        {
            upper++;
        }
        else if(str[i]>='a' && str[i]<='z')
        {
            lower++;
        }
    }
    printf("The Lowercase Letters are: %d\n",lower);
    printf("The Uppercase Letters are: %d\n",upper);


    return 0;

}