Fibonacci series gone wrong

I'm trying to print Fibonacci series in an file named "fibonacci.txt" up to 200 terms But after execution the file contained some wrong sums,

some sums were less than expected

• I've checked that it's not any wrong data type, I've used unsigned long so there should not be any problem.
• I'm using Cfree 5.0.

I'm attaching the code and output with question.

int main(){
ofstream text_file;
text_file.open("fibonnacci.txt");

unsigned long sum, sum1=1, sum2=1;

text_file<<sum1<<"\t"<<sum2<<"\t";
for(int i = 1; i < 200; i++)    {
    sum = sum1 + sum2;
    text_file<<sum<<"\n";
    /*if(i%5 == 0){
        text_file<<"\n";
    }*/
    sum2 = sum1;
    sum1 = sum;

}
text_file<<"\n";

return 0;
}

OUTPUT

102334155
165580141
267914296
433494437
701408733
1134903170
1836311903
2971215073
512559680
3483774753
3996334433
3185141890
2886509027
1776683621
368225352
2144908973
2513134325
363076002
2876210327
3239286329
1820529360
764848393
2585377753
3350226146

These are some mid terms. Clearly we can see that some terms are less than the previous terms by calculating number of digits.