p1914 60分 求助

#include<bits/stdc++.h>
using namespace std;
int n;
string a;
int b;
int c;
int main()
{
	cin>>n;
	cin>>a;
	for(int i = 0;i <= a.size() - 1;i++)
	{
		b = a[i];
		if(a[i] == n)
		{
			b = 97 + (n - 1);
			a[i] = b;
			cout<<a[i];
		}
		else
		{
			b += n;
			a[i] = b;
			cout<<a[i];
		}
	}
	return 0;
}