#include <stdio.h>
#include <string.h>
#define N 400
#define M (1 << 23)
#define INF 0x3f3f3f3f
int abs_(int a) { return a > 0 ? a : -a; }
int min(int a, int b) { return a < b ? a : b; }
int main() {
static char cc[N + 1];
static int kk[3][3][N], dp[M];
int n, i, nr, ng, ny, kr, kg, ky, c, u, v, x, ans;
scanf("%d%s", &n, cc);
nr = ng = ny = 0;
for (i = 0; i < n; i++)
if (cc[i] == 'R')
kk[0][1][nr] = ng, kk[0][2][nr] = ny, nr++;
else if (cc[i] == 'G')
kk[1][2][ng] = ny, kk[1][0][ng] = nr, ng++;
else
kk[2][0][ny] = nr, kk[2][1][ny] = ng, ny++;
memset(dp, 0x3f, (nr + 1) * (ng + 1) * (ny + 1) * 3 * sizeof *dp);
for (c = 0; c < 3; c++) {
u = ((0 * (ng + 1) + 0) * (ny + 1) + 0) * 3 + c;
dp[u] = 0;
}
for (kr = 0; kr <= nr; kr++)
for (kg = 0; kg <= ng; kg++)
for (ky = 0; ky <= ny; ky++)
for (c = 0; c < 3; c++) {
u = ((kr * (ng + 1) + kg) * (ny + 1) + ky) * 3 + c, x = dp[u];
if (x == INF)
continue;
if (kr < nr && c != 0) {
v = (((kr + 1) * (ng + 1) + kg) * (ny + 1) + ky) * 3 + 0;
dp[v] = min(dp[v], x + abs_(kk[0][1][kr] - kg) + abs_(kk[0][2][kr] - ky));
}
if (kg < ng && c != 1) {
v = ((kr * (ng + 1) + (kg + 1)) * (ny + 1) + ky) * 3 + 1;
dp[v] = min(dp[v], x + abs_(kk[1][2][kg] - ky) + abs_(kk[1][0][kg] - kr));
}
if (ky < ny && c != 2) {
v = ((kr * (ng + 1) + kg) * (ny + 1) + (ky + 1)) * 3 + 2;
dp[v] = min(dp[v], x + abs_(kk[2][0][ky] - kr) + abs_(kk[2][1][ky] - kg));
}
}
ans = INF;
for (c = 0; c < 3; c++) {
u = ((nr * (ng + 1) + ng) * (ny + 1) + ny) * 3 + c;
if (dp[u] != INF)
ans = min(ans, dp[u] / 2);
}
if (ans == INF)
ans = -1;
printf("%d\n", ans);
return 0;
}
