#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,sum=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		if(i%2==0){
			sum+=i; 
		}
  }
	cout<<sum;
	return 0;
}

用for循环解的

#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,sum=0,i=1;
	cin>>n;
	while(i<=n){
		if(i%2==0){
			sum+=i; 
		}
        i++;
	}
	cout<<sum;
	return 0;
}

用while循环解的

#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,sum=0,i=1;
	cin>>n;
	do{
		if(i%2==0){
			sum = sum+i; 
		}
        i++
	}while(i<=n)
	cout<<sum;
	return 0;
}

用do-while循环解的