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 for w in range(W)] for h in range(H)] for a in range(26)]
for a in range(26):
    for h in range(H):
        for w in range(W):
            if A[h][w]==a:
                dp[a][h][w]=0
            if h:
                dp[a][h][w]=min(dp[a][h][w],dp[a][h-1][w]+1)
            if w:
                dp[a][h][w]=min(dp[a][h][w],dp[a][h][w-1]+1)
            if h and w:
                dp[a][h][w]=min(dp[a][h][w],dp[a][h-1][w-1]+1)
ans=0
for h in range(H):
    for w in range(W):
        idx=[dp[a][h][w] for a in range(26) if dp[a][h][w]<=min(h,w)]
        idx.sort()
        idx.append(min(h,w)+1)
        if len(idx)>K:
            ans+=idx[K]-idx[K-1]
print(ans)