Python:

M=int(input('Nhap M: '))
k=0
s=0
while (s<=M):
    k=k+1
    s=s+k**2
print('k =',k-1)

C++:

#include <iostream>
#include <math.h>
using namespace std;
long M, k, s;
int main(){
    cout << "nhap M: ";
    cin >> M;
    while (s<=M){
        k++;
        s=s+pow(k,2);
    }
    cout << "k = " << k-1;
}

$\color{green}{\text{Cần code Java, C#, JavaScript thì nói để mình thêm}}$

$\color{red}{\text{#Daoanhviet96}}$

var t,m,i,k:longint;

begin

write('Nhap M: '); readln(m);

while t<=m do

begin

k:=i;

i:=i+1;

t:-=t+i*i;

end;

writeln('So can tim la: ',k);

readln;

end.