题解

#include<iostream>
using namespace std;
int main(){
	int s;
    cin>>s;
    if(s==1||s==2||s==3||s==4||s==5||s==6){
        cout<<s;
        return 0;
    }
    int a[1000100]={0,1,2,3,4,5,6};
    for(int i=6;i<=s;i++) {
        a[i]=a[i-1]-5+i;
    }
    cout<<a[s];
	return 0;
}

以下虽是标准答案，但提交不对

#include<iostream>
using namespace std;
int main(){
	int s;
    cin>>s;
    if(s==1||s==2||s==3||s==4||s==5){
        cout<<s;
        return 0;
    }
    int a[1000100]={0,1,2,3,4,5,6};
    for(int i=7;i<=s;i++) {
        a[i]=a[i-1]+a[i-5];
    }
    cout<<a[s];
	return 0;
}