def clear(lst):
    for r in lst:
        for j in range(S):
            r[j] = 0

def a2i(x): return ord(x) - 97

def score(lst,i,j):
    lst = sorted(lst)
    if K != 26: return lst[K] - lst[K-1]
    else: return min(i,j) - lst[K]

h,w,K = map(int,input().split())
b = [list(map(a2i,input())) for _ in range(h)]
S = 26
dp = [[0]*S for _ in range(w)]
ndp = [[0]*S for _ in range(w)]
ans = 0
for i in range(h):
    clear(ndp)
    for j in range(w):
        for k in range(S):
            ndp[j][k] = min(ndp[j-1][k],dp[j][k],dp[j-1][k])+1
        ndp[j][b[i][j]] = 0
    dp,ndp = ndp,dp
    for j in range(w):
        ans += score(dp[j],i,j)
print(ans)