#include <bits/stdc++.h>   using namespace std;   int n, ways[5050][5050], last[27], fat[5050], ifat[5050];   char s[5050];   int expo(int b, int e) {     if (!e) return 1;     if (e & 1)       return 1LL * b * expo(1LL * b * b % 1000000007, e >> 1) % 1000000007;     return expo(1LL * b * b % 1000000007, e >> 1);   }   int invmod(int x) { return expo(x, 1000000007 - 2); }   void prep() {     fat[0] = 1;     for (int i = 1; i < 5050; i++) fat[i] = 1LL * fat[i - 1] * i % 1000000007;     ifat[5050 - 1] = invmod(fat[5050 - 1]);     for (int i = 5050 - 1; i > 0; i--)       ifat[i - 1] = 1LL * ifat[i] * i % 1000000007;     assert(ifat[0] == 1);   }   long long nCr(int n, int k) {     return 1LL * fat[n] * ifat[k] % 1000000007 * ifat[n - k] % 1000000007;   }   int main() {     scanf( %d %s , &n, s + 1);     prep();     s[0] = char( a  + 26);     last[26] = n + 1;     ways[n + 1][0] = 1;     for (int i = n; i >= 0; i--) {       for (int c = 0; c < 27; c++) {         if (c == (s[i] -  a ) || (last[c] == 0)) continue;         for (int j = 0; j < (n - i + 1); j++) {           ways[i][j + 1] += ways[last[c]][j];           if (ways[i][j + 1] >= 1000000007) ways[i][j + 1] -= 1000000007;         }       }       last[s[i] -  a ] = i;     }     long long ans = 0;     for (int i = 2; i <= n + 1; i++) {       ans = (ans + nCr(n - 1, i - 2) * ways[0][i]) % 1000000007;     }     printf( %lld n , ans);     return 0;   }   