#include <iostream>
#include <string>

size_t foo(std::string const & str, char a, char b)
{
	std::string str2{a, b};
	size_t res = 0;
	size_t found = 0;
	while((found = str.find(str2, found))!=std::string::npos)
	{
		++res;
		found+=2;
	}
	return res;
}

int main() {
	std::string str = "abcabcabac";
	std::cout << foo(str, 'a', 'b') << std::endl;
	return 0;
}