C++ :

#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int main(){
    int n;
    cin>>n;
    int i,j,s=0;
    for(i=1;i<n;i++){
    	for(j=i+1;j<=n;j++){
    		if((i+j)%3==0||(i+j)%7==0){
    			s=s+1;
			}
		}
	}cout<<s<<endl;
}

Python :

n = int(input());
i = 0
x = 0
s = 0
for i in range(1,n+1):
    for x in range(i,n +1):
        if i != x:
            if (i + x) % 3 == 0 or (i + x) % 7 == 0:
                s += 1
print(s)