What does this error mean in C# ? Cannot implicitly convert type 'string' to 'System

I'm programming a calculator in visual C#.net for my high school's homework and I'm badly in need of help.
It's my program without code of numbers:
(btnP = +, btnZ = *, btnS = -, btnT=/)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Calculator
{
public partial class Form1 : Form
{
int x, y, c,d;
string e;
public Form1()
{
InitializeComponent();
}
private void btnE_Click(object sender, EventArgs e)
{
if (d==1)
{
y = Convert.ToInt32(textBox1.Text);
c = x + y;
e = Convert.ToString(c);
textBox1.Text += e ;

}
if (d == 2)
{
y = Convert.ToInt32(textBox1.Text);
c = x * y;
e = Convert.ToString(c)
textBox1.Text += e;
}
if (d == 3)
{
y = Convert.ToInt32(textBox1.Text);
c = x - y;
e = Convert.ToString(c);
textBox1.Text += e;
}
if (d == 4)
{
y = Convert.ToInt32(textBox1.Text);
c = x / y;
e = Convert.ToString(c);
textBox1.Text += e;
}
}

private void btnP_Click(object sender, EventArgs e)
{
d = 1;
x = Convert.ToInt32(textBox1.Text);
textBox1.Text = "";

}
private void btnZ_Click(object sender, EventArgs e)
{
d = 2;
x = Convert.ToInt32(textBox1.Text);
textBox1.Text = "";

}

private void btnN_Click(object sender, EventArgs e)
{
d = 3;
x = Convert.ToInt32(textBox1.Text);
textBox1.Text = "";
}

private void btnT_Click(object sender, EventArgs e)
{
d = 4;
x = Convert.ToInt32(textBox1.Text);
textBox1.Text = "";
}
}
}

This program has error in all parts witch is wrote Convert.ToString(c);
It's error:
Cannot implicitly convert type 'string' to 'System.EventArgs'
What's the problem?What should I do?
please help me !