#include <stdio.h>
#define N 4100
#define M 4100
#define MD 0x7fffffff
#define X 12345
#define Y 54321
int px[N], py[N];
void init() {
int i;
px[0] = py[0] = 1;
for (i = 1; i < N; i++) {
px[i] = (long long) px[i - 1] * X % MD;
py[i] = (long long) py[i - 1] * Y % MD;
}
}
int main() {
static char cc[N][M + 1];
static long long xx[N], yy[N];
int n, m, d_, h, i, j, x, y;
init();
scanf("%d%d%d", &n, &m, &d_);
for (i = 0; i < n; i++)
scanf("%s", cc[i]);
for (i = 0; i < n; i++) {
xx[i] = (long long) px[i] * d_ % MD;
yy[i] = (long long) py[i] * d_ % MD;
}
for (h = 0; h < m; h++) {
int ax, cx, gx, tx, ay, cy, gy, ty;
long long sx, sy;
ax = ay = cx = cy = gx = gy = tx = ty = 0;
for (i = 0; i < n; i++)
if (cc[i][h] == 'A')
ax = ((long long) ax + px[i]) % MD, ay = ((long long) ay + py[i]) % MD;
else if (cc[i][h] == 'C')
cx = ((long long) cx + px[i]) % MD, cy = ((long long) cy + py[i]) % MD;
else if (cc[i][h] == 'G')
gx = ((long long) gx + px[i]) % MD, gy = ((long long) gy + py[i]) % MD;
else if (cc[i][h] == 'T')
tx = ((long long) tx + px[i]) % MD, ty = ((long long) ty + py[i]) % MD;
sx = (long long) ax + cx + gx + tx, sy = (long long) ay + cy + gy + ty;
for (i = 0; i < n; i++)
if (cc[i][h] == 'A')
xx[i] = ((long long) xx[i] + sx - ax) % MD, yy[i] = ((long long) yy[i] + sy - ay) % MD;
else if (cc[i][h] == 'C')
xx[i] = ((long long) xx[i] + sx - cx) % MD, yy[i] = ((long long) yy[i] + sy - cy) % MD;
else if (cc[i][h] == 'G')
xx[i] = ((long long) xx[i] + sx - gx) % MD, yy[i] = ((long long) yy[i] + sy - gy) % MD;
else if (cc[i][h] == 'T')
xx[i] = ((long long) xx[i] + sx - tx) % MD, yy[i] = ((long long) yy[i] + sy - ty) % MD;
}
x = y = 0;
for (i = 0; i < n; i++)
x = (x + (long long) px[i] * d_) % MD, y = (y + (long long) py[i] * d_) % MD;
for (i = 0; i < n; i++)
if (xx[i] == x && yy[i] == y) {
for (j = 0; j < n; j++)
if (j != i) {
int d;
d = 0;
for (h = 0; h < m; h++)
if (cc[i][h] != cc[j][h])
if (d++ == d_)
goto out;
out:
if (d != d_)
break;
}
if (j == n) {
printf("%d\n", i + 1);
return 0;
}
}
return 0;
}
