write a program that will take 2 string from user and concatenate the 2nd string after first string

#include<stdio.h>
#include<string.h>
main()
{
    int i,j;
    char str1[100],str2[100];
    printf("Enter first string   ");
    gets(str1);/// str1 = "We"



    printf("Enter 2nd string   ");
    gets(str2);/// str1 = "are"

    strcat(str1,str2);/// weare

    for(i=strlen(str1),j=0;str2[i]!=NULL;i++,j++)///str2 = "are"  str1[]="are"
    {
        str1[i]= str2[j];
    }
    str1[i] = NULL;

    puts(str1); ///weare


}