What will be the output of the following C++ code?

#include <iostream>
#include <cstring>
using namespace std;
int main ()
{
    char str1[10] = "Hello";
    char str2[10] = "World";
    char str3[10];
    int  len ;
    strcpy( str3, str1);
    strcat( str1, str2);
    len = strlen(str1);
    cout << len << endl;
    return 0;
}