H,W,K=map(int,input().split())
A=[[ord(a)-97 for a in input()] for h in range(H)]
inf=1<<30
dp=[inf]*(26*W)
ans=0
for h in range(H):
    prev=dp
    dp=[inf]*(26*W)
    for w in range(W):
        for a in range(26):
            if A[h][w]==a:
                dp[a*W+w]=0
            if h:
                dp[a*W+w]=min(dp[a*W+w],prev[a*W+w]+1)
            if w:
                dp[a*W+w]=min(dp[a*W+w],dp[a*W+w-1]+1)
            if h and w:
                dp[a*W+w]=min(dp[a*W+w],prev[a*W+w-1]+1)
        idx=[dp[a*W+w] for a in range(26) if dp[a*W+w]<=min(h,w)]
        idx.append(min(h,w)+1)
        if len(idx)>K:
            idx.sort()
            ans+=idx[K]-idx[K-1]
print(ans)