按照题目意思用if做就行了

#include <bits/stdc++.h>
using namespace std;
int main(){
    freopen("bus.in","r",stdin);
    freopen("bus.out","w",stdout);
    int n;
    cin>>n;
    if(n < 200) cout<<n;
    else if(n<300) cout<<n+50;
    else if(n<500) cout<<n+100;
    else cout<<n+200;
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
int a;
int main(){
	freopen("bus.in", "r", stdin);
    freopen("bus.out", "w", stdout);
    cin>>a;
    if(a < 200){
        cout << a;
    }
    else if(a <= 300){
        cout << a + 50;
    }
    else if(a <= 500){
        cout << a + 100;
    }
    else{
        cout << a + 200;
    }
    return 0;
}

using namespace std;
int main(){
	freopen("bus.in", "r", stdin);
    freopen("bus.out", "w", stdout);
	long long n;
	cin>>n;
	if(n<200){
		n+=0;
	}else if(n>=200 && n<300){
		n+=50;
	}else if(n>=300 && n<500){
		n+=100;
	}else if(n>=500){
		n+=200;
	}
	cout<<n;
	return 0;
} 
**真不难**