Do this

string res = s;
FOR(i,n-1)res+=s;
cout<<res<<ln;

as for why s += s is not working. use the input 4 4 you'll see what is going wrong and somewhat like 26 26 will give the runtime

Looks like this exceeds some internal limit on the output size (~50 megabytes?) before anything else.

while you run s+=s it will increase your initial set of character exponential time, for example, 3,4:-Initial s="abcd' then s+=s gives s='abcdabcd' i.e., two times of original s, again after s+=s gives s='abcdabcdabcdabcd' i.e., 4 times of the original s but we need only 3 times of original s. so, use temporary_var=s for the first time and then, loop for (i, n) s+=temporary_var. if you use python you can do this in one line s=s*n or s*=n. I hope you understand the error.