import sys
import numpy as np


def main():
    buf = sys.stdin.buffer
    n = int(buf.readline())
    S = np.frombuffer(buf.readline().rstrip(), np.uint8)
    A = np.array([ord('A'), ord('C'), ord('G'), ord('T')])
    B = np.in1d(S, A)
    print(B.nonzero()[0][-1] + 1 if np.count_nonzero(B) else 0)


if __name__ == '__main__':
    main()