illegal operations

[aewarnick]

#include <stdio.h>
#include <iostream>
#include <cstdlib>
#include <iomanip>
using namespace std;

int main(int argc, char *argv[])
{
int R, F=R+1;
cout << "How many people are there? "; cin >> R;

int responses [20]= {0}; // initialize all memory locations to 0.
int frequency [21]= {0};

cout << "Enter scores: " << endl;

     for (int i=0; i<=R; i++){
     cin >> responses[i]; if (responses[i]<=-1) break;
     }
     for (int answer=0; answer <=R; answer++)
     ++frequency [responses[answer]]; // This is the key to it all!!!

cout << "Number" << setw(14) << "Frequency" << endl;

     for (int rating=1; rating <=F; rating++)
     cout<< rating << setw(11) << frequency[rating] << endl;

system("PAUSE");
return 0;
}

Every time I run this it loops up to 185 and then says it performed an illegal operation. Even if I put -1 as the first score.