h,w,k = map(int,input().split())
C = [[ord(x)-ord("a") for x in input()] for i in range(h)]
accum = [[[0]*26 for i in range(w+1)] for i in range(h+1)]
for i in range(h):
    for j in range(w):
        c = C[i][j]
        for t in range(26):
            accum[i+1][j+1][t] = accum[i+1][j][t]+accum[i][j+1][t]-accum[i][j][t]
        accum[i+1][j+1][c] += 1

def add(l,x,y,f):
    ll = accum[x][y]
    for i in range(26):
        l[i] += ll[i]*f
    return l

def calc(a,b,c,d):

    l = [0]*26
    l = add(l,c,d,1)
    l = add(l,a-1,b-1,1)
    l = add(l,c,b-1,-1)
    l = add(l,a-1,d,-1)
    num = 0
    for i in l:
        if i > 0:
            num += 1
    return num


ans = 0

for i in range(1,h+1):

    lx,ly = i,1
    rx,ry = i,1

    for j in range(i,h+1):
        x,y = j,(j-i)+1
        while lx < h+1 and calc(x,y,lx,ly) < k:
            lx,ly = lx+1,ly+1

        while rx < h+1 and calc(x,y,rx,ry) <= k:
            rx,ry = rx+1,ry+1

        if lx >= h+1:
            break
        if calc(x,y,lx,ly) == k:
            ans += rx-lx


for i in range(2,w+1):
    
    lx,ly = 1,i
    rx,ry = 1,i

    for j in range(i,w+1):
        x,y = (j-i)+1,j
        while ly < w+1 and calc(x,y,lx,ly) < k:
            lx,ly = lx+1,ly+1

        while ry < w+1 and calc(x,y,rx,ry) <= k:
            rx,ry = rx+1,ry+1

        if ly >= w+1:
            break
        if calc(x,y,lx,ly) == k:
            ans += rx-lx
print(ans)