How can I define a game over in snake game using SFML?

so recently I have been wanting to make a snake game in C++ using SFML in Microsoft Visual studio 2015 and I made one and I am actually pretty satisfied with my work but there is a problem, that I forgot to make a game over for it and it seems like I couldn't make it work and it really had me on edge. So I thought I could use stack overflow's help. I would really appreciate it if you guys would let me know how to make it work and please keep it simple obvious. Here is my code:

// GraphicalLoopSnakeGame.cpp : Defines the entry point for the console application.

#include "stdafx.h"
#include <SFML/Graphics.hpp>
#include <time.h>
using namespace sf;

int N = 30, M = 20;
int size = 16;
int w = size*N;
int h = size*M;

int dir, num = 4;

struct Snake
{
    int x, y;
}      s[100];

struct Fruit
{
    int x, y;
}     f;

void Tick()
{
    for (int i = num;i>0;--i)
    {
    s[i].x = s[i - 1].x; s[i].y = s[i - 1].y;
}

if (dir == 0) s[0].y += 1;
if (dir == 1) s[0].x -= 1;
if (dir == 2) s[0].x += 1;
if (dir == 3) s[0].y -= 1;

if ((s[0].x == f.x) && (s[0].y == f.y))
{
    num++; f.x = rand() % N; f.y = rand() % M;
}

if (s[0].x>N) s[0].x = 0;  if (s[0].x<0) s[0].x = N;
if (s[0].y>M) s[0].y = 0;  if (s[0].y<0) s[0].y = M;

for (int i = 1;i<num;i++)
    if (s[0].x == s[i].x && s[0].y == s[i].y)  num = i;
}

int main()
{
    srand(time(0));
    RenderWindow 
window(VideoMode(w, h), "Snake Game!");

    Texture t1, t2, t3;

t1.loadFromFile("images/white.png");

  t2.loadFromFile("images/red.png");

   t3.loadFromFile("images/green.p 
ng");

    Sprite sprite1(t1);
    Sprite sprite2(t2);
    Sprite sprite3(t3);

    Clock clock;
    float timer = 0, delay = 0.13;

    f.x = 10;
    f.y = 10;

   while (window.isOpen())
    {
        float time = 
clock.getElapsedTime().asSeconds();
        clock.restart();
        timer += time;

        Event e;
        while 
(window.pollEvent(e))
        {
            if (e.type == Event::Closed)

   window.close();
        }

    if (Keyboard::isKeyPressed(Keyboard::Left)) dir = 1;
    if (Keyboard::isKeyPressed(Keyboard::Right)) dir = 2;
    if (Keyboard::isKeyPressed(Keyboard::Up)) dir = 3;
    if (Keyboard::isKeyPressed(Keyboard::Down)) dir = 0;
    if (Keyboard::isKeyPressed(Keyboard::W)) dir = 3;
    if (Keyboard::isKeyPressed(Keyboard::D)) dir = 2;
    if (Keyboard::isKeyPressed(Keyboard::A)) dir = 1;
    if (Keyboard::isKeyPressed(Keyboard::S)) dir = 0;

    if (timer>delay) { timer = 0; Tick(); }

    ////// draw  ///////
    window.clear();

    for (int i = 0; i<N; i++)
        for (int j = 0; j<M; j++)
        {
            sprite1.setPosition(i*size, j*size);  window.draw(sprite1);
        }

    for (int i = 0;i<num;i++)
    {
        sprite2.setPosition(s[i].x*size, s[i].y*size);  window.draw(sprite2);
    }

    sprite3.setPosition(f.x*size, f.y*size);  window.draw(sprite3);

    window.display();
}

return 0;
}