Convert any input of length measurement into meters C++

[hughng92]

Given:
1 foot = 0.305 m
1 in = 0.0254m.
I am attempting to convert the input of length measurements into meters.
For example: 5'1'' = 5*0.305 + 1 * 0.0254 = 1.5504m
3.4m = 3.4m
5'1.23456" = 5*0.305 + 1.23456*0.0254 = 1.5563
But I have got quite a few errors. I don't know how to fix them but here is my logics on how to approach this puzzle.
I wrote some if statements to take care of:
1) If I input only, say 3.4, it will print the whole string and convert it into double 3.4.
2) If I input, say 5'1" without no "."
)If I input, say 5'1.23" with " . ",
But I failed. I need your help to point out the problems of my code. Thanks in advance!

#include <iostream>
#include <fstream>
#include <string>
#include <cstring>
#include <sstream>

using namespace std;

double convert_String(string&);

int main()
{
	string s;
	getline(cin,s);
	convert_String(s);
	return 0;

}

double convert_String(string& s)
{
	string s1,s2;
	int y1, y2,y3;
	double d1,d2,d3;
	int l = s.length();
	y1 = s.find("."); // find the position of the "." and then assign the position value to y1
	y2 = s.find("'"); // find the position of the " ' " and then assign the position value to y
	y3 = s.find('"'); // find the position of the " " " and then assign the position value to y2
	if ((y1 < 0 && y2 < y1 && y3 == l) || (y2 < 0 && y3 < 0 && y1 == -1))
	{
		s1 = s.substr(0,y2-1);
		d1 = stod(s1);
		d1 = d1*0.305;
		s2 = s.substr(y2+1,l-1);
		d2 = stod(s2);
		d2 = d2*0.0254;
	}
	else if(y2 == -1 && y3 == -1 && y1 >= 0) //there is no " ' "  and " " "
	{
		d3 = stod(s);
	}
    cout << d1 << endl;
	return d1 + d2;
}