萌新求助，我这样写有什么问题？为什么每次find函数返回值都是-1？？？

#include<cstdio>
#include<string>
using namespace std;

int main()
{
	string three = "abcdefghijklmnotuv", four = "pqrswxyz";
	char a;
	int cnt = 0;
	while(a = getchar() != EOF)
	{
		if(three.find(a) != -1)
			cnt += (three.find(a) % 3);
		else if(four.find(a) != -1)
			cnt += four.find(a) % 4;
		else
			cnt++;
	}
	printf("%d", cnt);
	
	return 0;
}