C++ :

#include <iostream>
#include <string.h>
using namespace std;
int total = 0;
void fun(int n, char* str)
{
    if(n==0)
    {
        char* p1 = strstr(str,"111");
        //char* p2 = strstr(str,"000");
        if(p1 == NULL)
        {
            total++;
        }
    }
    else{
        str[n-1]='1';
        fun(n-1,str);
        str[n-1]='0';
        fun(n-1,str);
        }
}

int main()
    {
        char str[41]={0};
        int n;
        scanf("%d",&n);
        fun(n,str);
        cout<<total<<endl;
        return 0;
    }