Why I've got MLE(19 MB). I use KMP and I don't understand why at all! I appeal to you for help...
#include<iostream>
#include<string>
#include<vector>
using namespace std;
int main() {
  int  i, j, f=0, n;
  string what_to_find, gde;
  cin>>n>>what_to_find>>gde;
  what_to_find+="#";
  what_to_find+=gde;
  what_to_find+=gde;
  gde="";
  vector<short> next(what_to_find.size());
  next[0]=0;
  for(i=1; i<what_to_find.size(); i++){
    if(what_to_find[i]==what_to_find[next[i-1]])
      next[i]=next[i-1]+1;
    else {
      j = next[i-1];
      while (j>0 && what_to_find[i]!=what_to_find[j]){
      j=next[j-1];
      if(what_to_find[i]==what_to_find[j])
        next[i]=j+1;
      }
    }
    if(next[i]==n) {
      f=1;
      break;
    }
  }

  if(f) cout<<i-2*n;
  else cout<<-1;
  return 0;

}

When I was using strings,I had MLE 8. Bit in my AC programm I used arrays of char.

I get MLE#8 too, using std::string but my AC program with arrays uses only 4.4 MB...
Admins, can you please explain such strange behaviour of the judge system?

Try to preallocate strings:

int n; cin >> n >> ws;
string str(n,' ');
getline(cin, str);