### BUGGY CODE ###

n = int(input())
s = input()
bc = len(s)*[0]
gc = len(s)*[0]
rc = len(s)*[0]
for i in range(1, len(s)+1):
    mi = len(s) - i
    if s[len(s) - i] == 'B':
        if i == 1:
            bc[mi] = 1
        else:
            bc[mi] = bc[mi+1] + 1
            rc[mi] = rc[mi+1]
            gc[mi] = gc[mi+1]
    elif s[mi] == 'R':
        if i == 1:
            rc[mi] = 1
        else:
            rc[mi] = rc[mi+1] + 1
            bc[mi] = bc[mi+1]
            gc[mi] = gc[mi+1]
    elif s[mi] == 'G':
        if i ==1:
            gc[mi] = 1
        else:
            gc[mi] = gc[mi+1] + 1
            rc[mi] = rc[mi+1]
            bc[mi] = bc[mi+1]
si = 0
for i in range(0, len(s) - 1):
    
    for j in range(i+1, len(s)-1):
        if s[j] != s[i]:
            d = j-i 
            if j+d < n:
                nc = s[j+d]
                if nc in [s[i], s[j]]:
                    l = list(set(['R', 'G', 'B']) - set([s[i], s[j]]))
                    if l[0] == 'R':
                        si+= rc[j+1]
                    elif l[0] == 'G':
                        si += gc[j+1] 
                    else:
                        si += bc[j+1]
                else:
                    l = list(set(['R', 'G', 'B']) - set([s[i], s[j]]))
                    if l[0] == 'R':
                        si+= rc[j+1]-1
                    elif l[0] == 'G':
                        si += gc[j+1] -1
                    else:
                        si += bc[j+1] - 1
            else:
                l = list(set(['R', 'G', 'B']) - set([s[i], s[j]]))
                if l[0] == 'R':
                    si+= rc[j+1]
                elif l[0] == 'G':
                    si += gc[j+1] 
                else:
                    si += bc[j+1]
print(si)
                
  
  
        
            

### FIXED CODE ###

n = int(input())
s = input()
bc = n*[0]
gc = n*[0]
rc = n*[0]
for i in range(1, n+1):
    mi = n - i
    if s[n - i] == 'B':
        if i == 1:
            bc[mi] = 1
        else:
            bc[mi] = bc[mi+1] + 1
            rc[mi] = rc[mi+1]
            gc[mi] = gc[mi+1]
    elif s[mi] == 'R':
        if i == 1:
            rc[mi] = 1
        else:
            rc[mi] = rc[mi+1] + 1
            bc[mi] = bc[mi+1]
            gc[mi] = gc[mi+1]
    elif s[mi] == 'G':
        if i ==1:
            gc[mi] = 1
        else:
            gc[mi] = gc[mi+1] + 1
            rc[mi] = rc[mi+1]
            bc[mi] = bc[mi+1]
si = 0
for i in range(0, n - 1):
    
    for j in range(i+1, n-1):
        if s[j] != s[i]:
            d = j-i 
            if j+d < n:
                nc = s[j+d]
                if nc in [s[i], s[j]]:
                    l = list(set(['R', 'G', 'B']) - set([s[i], s[j]]))
                    if l[0] == 'R':
                        si+= rc[j+1]
                    elif l[0] == 'G':
                        si += gc[j+1] 
                    else:
                        si += bc[j+1]
                else:
                    l = list(set(['R', 'G', 'B']) - set([s[i], s[j]]))
                    if l[0] == 'R':
                        si+= rc[j+1]-1
                    elif l[0] == 'G':
                        si += gc[j+1] -1
                    else:
                        si += bc[j+1] - 1
            else:
                l = list(set(['R', 'G', 'B']) - set([s[i], s[j]]))
                if l[0] == 'R':
                    si+= rc[j+1]
                elif l[0] == 'G':
                    si += gc[j+1] 
                else:
                    si += bc[j+1]
print(si)