C++,0分，求助

#include<iostream>
#include<string>
using namespace std;
int l,i;
int main() {
	string a;
	getline(cin,a);
	l=a.size();
	for(i=0; i<l; i++)
		if((i+1)%2==0) 
			if(a[i]>='A'&&a[i]<='Z') a[i]=a[i]-'A'+'a';
        else 
			if(a[i]>='a'&&a[i]<='z') a[i]=a[i]-'a'+'A';
	cout<<a;
	return 0;
}
//输入i like eat apple，输出i lIkE EaT ApPlE